MANUAL 


OF  THE 


CHEMICAL  ANALYSIS 
OF  ROCKS 


BY 

HENRY  S.  WASHINGTON,  Ph.l). 


THIRD  EDITION,  REVISED  AND  ENLARGED 


NEW  YORK 

JOHN  WILEY  &   SONS,   INC. 

LONDON:  CHAPMAN  &   HALL,   LIMITED 

1919 


QE4 
W3 


Copyright,  1904,  1910,  1919 

BY 

H.  S.  WASHINGTON. 


First  Edition — Entered  at  Stationers'  Hall. 


PRESS  OP 

8RAUNWORTH  &  CO, 

BOOK   MANUFACTURER? 

BROOKLYN.  N.  Y. 


PREFACE   TO   THE   THIRD   EDITION 


THE  present  edition  has  been  thoroughly  revised  and  consid- 
erably enlarged.  Some  changes  in  details  of  procedure  have  been 
introduced  that  were  suggested  by  the  equipment  and  facilities 
of  a  modern  laboratory,  as  contrasted  with  those  which  served  for 
the  work  upon  which  the  previous  editions  were  based.  But  the 
endeavor  has  been  made  to  describe  the  various  procedures  so 
that  they  may  be  readily  carried  out  by  one  working  in  a  laboratory 
that  is  not  provided  with  all  possible  facilities.  Some  new  methods 
have  been  introduced,  but  it  has  been  thought  to  be  the  wiser 
course  to  adhere  to  well-known  and  oft-tried  reliable  methods, 
rather  than  to  supplant  them  by  others  more  recently  proposed, 
unless  these  have  been  tried  and  proved  to  be  of  undoubted  supe- 
riority. 

The  various  methods  have  been  treated  in  much  greater 
detail  than  in  previous  editions,  and  more  stress  has  been  laid 
on  the  sources  of  error  both  in  operations  and  in  methods;  expe- 
rience has  shown  that  such  discussions  will  be  of  service  to  the 
intelligent  student,  particularly  if  he  is  working  alone.  It  is- 
to  be  hoped  that  the  many  sources  of  error,  and  the  many  pre- 
cautions, which  have  been  enumerated,  will  not  induce  discour- 
agement at  the  outset,  but  that  they  will,  rather,  serve  to  stim- 
ulate to  greater  and  more  active  and  intelligent  interest  in  and 
understanding  of  the  analysis  of  rocks. 

Some  of  the  most  recent  and  standard  text-books  have  been 
at  my  elbow  during  the  revision.  The  work  throughout  is  greatly 
indebted,  as  before,  to  the  invaluable  writings,  as  well  as  to  the 
friendly  advice,  of  Dr.  W.  F.  Hillebrand;  while  the  treatise  of 
Dr.  J.  W.  Mellor  has  also  been  of  great  assistance  and  has  been 
constantly  consulted. 

It  is  a  pleasure  to  express  my  obligations  to  Dr.  Arthur  L. 

iii 


394156 


iv  PREFACE  TO  THE  THIRD  EDITION 

Day,  Director  of  the  Geophysical  Laboratory,  for  his  permission 
to  undertake  this  revision  while  a  member  of  its  staff;  and  I 
would  also  tender  my  thanks  to  Dr.  E.  G.  Zies,  of  this  laboratory, 
for  much  friendly  and  valuable  criticism  and  counsel,  as  well  as  for 
his  kindness  in  reading  the  proofs  during  my  absence. 

H.  S.  W. 

GEOPHYSICAL  LABORATORY, 
Carnegie  Institution  of  Washington, 
June  30,  1918. 


PREFACE   TO   THE   FIRST  EDITION 


THE  object  of  this  book  is  to  present  to  chemists,  petrologists, 
mining  engineers  and  others  who  have  not  made  a  particular 
study  of  quantitative  analysis,  a  selection  of  methods  for  the 
chemical  analysis  of  silicate  rocks,  and  especially  those  of 
igneous  origin.  While  the  publication  of  such  a  work  may  seem 
superfluous  in  view  of  the  existence  of  Hillebrand's  treatise  on 
this  special  topic,  yet  justification  may  be  found  in  the  fact  that 
the  latter  is  intended,  not  so  much  for  one  who  is  not  very  con- 
versant with  the  subject,  as  for  the  practiced  analyst,  to  whom 
it  is  an  indispensable  guide. 

A  further  reason  for  its  appearance  is  that,  apart  from  Hille- 
brand's  book  and  a  paper  by  Dittrich,  there  does  not  seem  to 
exist  any  separate  modern  treatise  on  the  chemical  analysis  of 
rocks.  The  space  devoted  to  this  branch  of  analysis  in  the  text- 
books is  usually  very  small,  and  the  various  methods  are  widely 
scattered  and  often  inadequately  described.  This  is  especially 
true  in  regard  to  the  minutiae  of  manipulation  and  precautions 
to  be  observed,  and  to  the  determination  of  elements  which, 
though  usually  accounted  rare,  have  of  late  years  been  shown 
to  be  very  common  rock  constituents.  This  neglect  is  rather 
striking  in  view  of  the  prominence  given  in  the  last  decade  or  so 
to  the  chemical  composition  of  igneous  rocks. 

There  is  an  increasing  number  of  geologists,  petrologists, 
chemists  and  others,  who  are  desirous  of  making  chemical  anal- 
yses of  rocks,  but  who  have  had  little  or  no  experience  in  the 
subject,  except  that  gained  in  the  ordinary  course  of  quantita- 
tive analysis,  in  which  the  study  of  silicates  is  usually  confined 
to  the  examination  of  a  feldspar  or  some  such  simple  mineral. 
It  is  for  the  benefit  of  this  class  of  students  that  the  present 
book  is  written.  The  general  plan  adopted  therefore  is,  not  to 
attempt  a  complete  treatise  on  rock  analysis,  but  to  present  only 


vi  PREFACE  TO  THE  FIRST  EDITION 

certain  methods  which  have  proved  simple  and  reliable  in  the 
experience  of  the  chemists  of  the  U.  S.  Geological  Survey  and 
of  my  own.  The  more  important  of  these,  and  some  of  the  prin- 
cipal operations,  are  described  with  great  explicitness.  Many 
small  details  of  manipulation  are  gone  into  which  are  omitted  by 
Hillebrand  and  the  text-books  as  unnecessary,  a  knowledge  of 
them  being  either  presupposed  or  their  demonstration  left  to  the 
instructor. 

In  this  way  it  is  hoped  that  it  will  be  possible  for  an  intel- 
ligent student,  with  some  knowledge  of  chemistry  and  a  little 
analytical  training,  to  be  able  to  complete  a  satisfactory  analysis 
of  an  ordinary  silicate  rock,  without  personal  instruction  and 
after  comparatively  short  practice.  To  the  expert  analyst, 
therefore,  the  book  will  contain  much  that  is  superfluous,  but  for 
this  no  apology  is  offered.  What  are  superfluities  to  him  will, 
it  is  hoped,  be  welcome  to  the  novice. 

It  is  assumed  that  silicate  igneous  rocks  will  be  the  most  fre- 
quent objects  of  investigation.  At  the  same  time,  the  methods 
described  serve  equally  well  for  most  silicate  metamorphic  and 
sedimentary  rocks.  Such  rocks  as  saline  deposits,  coals  and 
others  containing  organic  matter  are  not  considered.  The 
methods  are  not  generally  adapted  to  the  analysis  of  ores  which, 
with  such  constituents  as  sulphides,  arsenides  and  other  com- 
pounds of  the  heavy  metals,  often  call  for  different  and  more 
complex  means  of  separation  than  are  here  given.  The  same 
is  true  of  many  minerals,  though  the  methods  found  in  the  fol- 
lowing pages  are  those  appropriate  to  the  analysis  of  most  silicates. 
The  analysis  of  meteorites  also  demands  the  employment  of  special 
methods,  and  in  most  cases  these  bodies  are  of  such  character  that 
their  examination  should  not  be  undertaken  by  the  inexperienced, 
especially  if  only  a  limited  supply  of  material  is  available. 

The  methods  selected  are,  in  general,  those  adopted  by  the 
chemists  of  the  U.  S.  Geological  Survey,  and  which  in  their 
essentials  I  have  employed  in  my  own  scientific  work  for  a  number 
of  years.  Some  modifications  have  been  made,  chiefly  in  the 
direction  of  simplification  and  the  elimination  of  certain  refine- 
ments which  do  not  seem  called  for  when  the  object  of  the  volume 
is  considered.  There  is  no  attempt  at  the  introduction  of  new 
methods  or  the  description  of  alternative  ones  which,  either  on 


PREFACE  TO  THE  FIRST  EDITION  vii 

theoretical  grounds  or  on  account  of  practical  difficulties,  are 
deemed  to  be  less  well  adapted  to  the  needs  of  students  than 
those  which  are  here  given.  Theoretical  discussion  will  be  lim- 
ited to  what  may  seem  necessary  to  make  clear  the  principles  of 
certain  methods  or  the  reasons  for  their  selection. 

I  have  also  endeavored  to  point  out  to  the  student  the  im- 
portance of  chemical  analyses  for  the  study  of  rocks,  and  their 
possible  bearing  on  some  of  the  broad  problems  which  form  the 
objects  of  the  science  of  petrology.  In  other  words,  it  has  been 
sought  to  emphasize  the  fact  that  petrographical  classifications 
and  the  study  of  textures  and  of  minerals  in  thin  sections  are  not 
the  sole  aims  of  the  science,  but  that,  supplemented  by  a  knowl- 
edge of  the  chemical  composition  of  igneous  rocks,  they  are  only 
means  to  broader  ends.  I  can  only  express  the  hope  that  this 
little  book  will  aid  in  the  progress  of  petrology,  by  leading  to 
an  increase  in  the  knowledge  of  chemical  analysis  among  petrol- 
ogists  and  rendering  our  data  in  the  way  of  rock  analyses  of 
superior  quality  more  numerous. 

The  great  obligations  under  which  I  am  to  Dr.  Hillebrand's 
work  are  evident  throughout  and  are  most  gratefully  acknowl- 
edged. The  text-books  of  Fresenius,  Classen,  Treadwell,  and 
Jannasch  have  also  been  consulted,  and  the  book  is  indebted 
to  them  in  many  ways.  It  is  also  a  pleasure  to  express  my  obli- 
gations to  several  friends  for  valuable  advice  and  assistance,  and 
especially  to  Prof.  S.  L.  Penfield  and  Prof.  L.  V.  Pirsson,  to  whom 
my  first  knowledge  of,  and  training  in,  quantitative  analysis  are 
due.  A  number  of  most  useful  hints  in  manipulation  were  learned 
from  these  two  analysts,  all  of  which  could  not  be  specifically 
mentioned  in  their  proper  places,  but  which  are  acknowledged  here. 
Acknowledgments  are  also  due  to  the  Trustees  of  the  Carnegie 
Institution  for  permission  to  publish  an  analysis  made  under  their 
auspices. 

The  factors  used  in  calculations  are  those  given  by  Cohn  in  his 
recent  translation  of  Fresenius'  Quantitative  Analysis.  All  tem- 
peratures are  given  in  centigrade  degrees.  The  metric  system  is  used 
generally,  except  in  dealing  with  such  pieces  of  apparatus  as  are 
usually  sold  in  this  country  on  the  basis  of  English  measurements. 

HENRY  S.  WASHINGTON. 
LOCUST,  N.  J.,  May,   1904. 


CONTENTS 


PART  I 
INTRODUCTION 

PAGE 

1.  Importance  of  Chemical  Analyses 1 

2.  General  Character  of  Analyses 3 

Accuracy  of  Analyses 3 

Completeness  of  Analyses 5 

3.  Microscopical  Examination 6 

4.  Constituents  to  be  Determined 7 

Main  Constituents 11 

Minor  Constituents 13 

5.  The  Occurrence  of  Various  Elements 17 

6.  Statement  of  Analyses 21 

PART  II 

APPARATUS  AND  REAGENTS 

1.  Apparatus 27 

Balance  and  Weights 27 

Balance 27 

Weights 29 

Platinum 30 

List  of  Apparatus 31 

Care  of  Platinum 32 

Glass 34 

Fused  Silica 39 

Porcelain 39 

Rubber '. * 40 

Metal 40 

Miscellaneous 43 

2.  Reagents : 45 

PART  III 

THE  SAMPLE 

1.  Selection  in  the  Field 57 

Uniformity  of  the  Rock-mass 58 

Freshness  of  the  Rock 59 

ix 


CONTENTS 


2.  Amount  of  Material 62 

3.  Preparation  of  the  Sample 63 

Sampling 63 

Methods  of  Pulverization 64 

Pulverization  of  the  Sample 68 

PART  IV 

OPERATIONS 

1.  Preliminary  Observations 73 

2.  Sources  of  Operative  Errors 75 

3.  Weighing 79 

4.  Decomposition 84 

5.  Precipitation 87 

6.  Filtration  and  Washing 90 

Simple  Filtration 91 

Washing  of  Precipitates 96 

Suction  Filtration 98 

Gooch  Crucible 99 

7.  Drying  and  Ignition 101 

Drying 101 

Ignition 103 

8.  Titration 105 

Volume  Burette 106 

Weight  Burette 107 

The  Operation 108 

PART  V 

METHODS 

1.  General  Course  of  Analysis 109 

2.  Time  Needed  for  an  Analysis 113 

3.  Errors  and  Summation 119 

Character  of  Errors 119 

Direction  of  Errors 120 

Limit  of  Error 124 

Summation 126 

4.  Weighing  out  the  Portions 129 

5.  Fusion  with  Sodium  Carbonate 131 

.       The  Fusion , . ; 131 

Removal  of  the  Cake 135 

Solution  of  the  Cake 137 

6.  Silica 139 

Errors 139 

Separation  of  Silica .-• 140 

Ignition  of  Silica 143 


CONTENTS  xi 


7.  Alumina  Precipitate 146 

Errors  in  Alumina 147 

Precipitation  by  Ammonia 150 

"Basic  Acetate"  Precipitation 155 

Ignition  of  the  Precipitate 157 

Fusion  with  Pyrosulphate 159 

8.  Total  Iron  Oxides 162 

Errors 162 

Reduction  of  Ferric  to  Ferrous  Oxide 163 

Titration  of  Iron 166 

9.  Titanium  Dioxide 167 

Colorimetric  Method 168 

Errors 168 

The  Operation 169 

Gravimetric  Methods 175 

10.  Lime  and  Strontia 177 

Errors 177 

Precipitation 178 

Strontia 179 

11.  Magnesia 180 

Errors 180 

Precipitation 181 

12.  Ferrous  Oxide 182 

Errors 183 

Simple  Method 186 

Pratt's  Method 190 

13.  Potash  and  Soda 191 

Errors 192 

Smith  Method -. 193 

Separation  of  Potash 202 

Separation  as  Platinichloride 203 

Separation  as  Perchlorate 207 

Determination  of  Potash  Alone 208 

14.  Hygroscopic  Water 208 

15.  Combined  Water 210 

Errors 210 

Penfield's  Method 213 

16.  Phosphorus  Pentoxide 216 

Errors 216 

Precipitation  as  Phosphomolybdate 216 

17.  Manganous  Oxide 219 

Errors 220 

Colorimetric  Method 220 

Gravimetric  Method 223 

18.  Sulphur,  Zirconia,  Baryta,  and  Rare  Earths 225 

Decomposition 225 


xii  CONTENTS 

PAGE 

Sulphur 226 

Zirconia 227 

Baryta 229 

Rare  Earths 229 

19.  Sulphur  Trioxide 231 

20.  Chlorine 232 

21.  Fluorine 233 

22.  Carbon  Dioxide 235 

23.  Chromium  and  Vanadium 237 

24.  Copper  and  Nickel 238 

25.  Boric  Oxide 240 

APPENDIXES 

1.  Factors  for  Calculation 241 

2.  Example  of  Analysis 242 

3.  References 247 

INDEX.  .                                                                                               .  249 


THE  CHEMICAL  ANALYSIS  OF  ROCKS 


PART   I 

INTRODUCTION 

1.  IMPORTANCE  OF  CHEMICAL  ANALYSES 

FOR  the  greater  part  of  a  century,  since  their  study  began, 
igneous  rocks  were  regarded  almost  solely  as  more  or  less  for- 
tuitous mineral  aggregates,  these  being  usually  assumed  to  be 
due  to  the  fusion  of  previously  existent  rock  bodies  or  to  the 
mixture  of  several  igneous  magmas.  With  the  introduction  of 
the  microscope,  a  more  intimate  study  of  their  field  relations,  and 
especially  with  the  improved  chemical  methods  and  the  greatly 
increased  number  of  satisfactory  chemical  analyses  of  the  last 
thirty  years,  a  decided  change  has  come  about  in  the  way  of 
regarding  them. 

Various  observations  and  theories  of  the  order  of  succession 
and  of  crystallization  of  minerals,  differentiation  of  bodies  of 
magma,  consanguinity  and  petrographic  provinces,  have  been 
made  and  advanced,  and  the  principles  of  physical  chemistry 
have  been  applied  to  their  study;  all  these  tending  to  throw  light 
on  the  origin,  genetic  relationships  and  mode  of  formation  of  igne- 
ous rocks.  Briefly  put,  the  tendency  of  the  modern  study  of 
igneous  rocks  is  toward  considering  them  as  falling  under  Spencer's 
law  of  evolution;  that  is,  in  the  general  line  of  passage  from  "  an 
indefinite  incoherent  homogeneity  to  a  definite,  coherent  hetero- 
geneity." In  other  words,  the  petrologist  of  the  present  day 
does  not  regard  them  as  merely  solidified  mineral  aggregates,  whose 
characters  are  largely  the  result  of  chance  conditions,  but  as  bodies 
which  are  the  result  of  the  action  of  physico-chemical  proc- 


2  :  INTRODUCTION 

esses,  and  whose  characters  are  determined  by  evolutionary  laws. 
It  is  the  aim  of  petrology  to  interpret  these  pieces  of  evidence  and 
to  ascertain  the  laws  which  govern  the  origin  and  formation  of 
rocks.  It  is  needless  to  say  that  this  modern  point  of  view  renders 
igneous  rocks  objects  of  far  greater  scientific  interest  than  they 
could  have  been  under  the  older  one. 

For  the  proper  study  and  understanding  of  these  theoretical 
aspects  of  igneous  rocks,  the  knowledge  and  application  of  some 
of  the  principles  of  physical  chemistry  are  necessary,  and  it  is 
obvious  that  for  this  a  detailed  knowledge  of  their  chemical  com- 
position, as  well  as  of  their  field  relations,  is  essential.  Conversely, 
it  seems  probable  that  the  study  of  igneous  rocks  will  be  of  service 
to  the  sister  science  of  physical  chemistry,  since  the  petrologist  is 
dealing  with  solidified  solutions  which  have  been  formed  and  acted 
on  by  physico-chemical  forces,  under  conditions  of  temperature, 
pressure,  and  mass  which  it  is  now  impossible  to  reproduce  in  the 
laboratory. 

To  the  petrographer,  who  deals  especially  with  the  descrip- 
tive and  systematic  portions  of  the  science  of  rocks,  the  chemical 
analysis  of  igneous  rocks  is  assuming  each  year  an  increasing  im- 
portance for  their  classification.  Whether  this  is  based  only  on 
the  inherent  characters  of  the  rock-mass  itself,  or  whether  it  takes 
account  of  genetic  relationships,  the  chemical  composition  is 
becoming  more  and  more  an  essential  factor,  and  one  which  can 
no  longer  be  relegated  to  the  background,  behind  the  superficially 
more  prominent  features  of  mode  of  occurrence,  texture,  and  qual- 
itative mineral  composition. 

While  our  knowledge  of  metamorphic  rocks  is,  as  yet,  not 
so  far  advanced  as  that  of  the  igneous  ones,  chemical  composition 
plays,  likewise,  a  most  important  part  in  their  study  and  classifi- 
cation, and,  to  a  certain  extent,  the  same  is  true  of  the  sedimentary 
rocks. 

As  regards  the  economic  side  of  geology,  such  as  the  origin 
and  formation  of  ores  and  useful  mineral  deposits,  there  is  accu- 
mulating evidence  of  the  importance  of  a  knowledge  of  the  chem- 
ical composition  of  igneous  and  metamorphic  rocks.  This  refers 
not  only  to  their  main  features,  but  also  to  the  occurrence  in  them 
of  the  less  abundant  elements,  which  by  certain  processes  of  seg- 
regation may  become  commercially  available. 


GENERAL  CHARACTER  OF  ANALYSES         3 

It  is  therefore  evident  that  we  possess  in  chemical  analysis 
a  means  of  investigation  that  complements,  and  is  of  value  fully 
commensurate  with,  the  study  of  rocks  in  the  field  or  with  the 
microscope.  That  this  is  generally  recognized  is  shown  by  the 
increasing  prominence  given  to  chemical  analyses  in  recent 
petrological  and  petrographical  papers,  as  well  as  in  publica- 
tions of  an  economic  character.  It  is  also  shown  by  the  attention 
given  to  this  study  by  official  organizations,  and  by  the  growing 
number  of  those  who  make,  or  who  desire  to  make,  analyses  of  rocks. 

2.  GENERAL  CHARACTER  OF  ANALYSES  x 

For  a  fuller  understanding  of  the  general  subject,  it  will  be 
well  to  discuss  briefly  the  factors  which  make  up  the  character  of 
a  rock  analysis,  and  which  determine  its  value. 

The  fulfilment  of  two  conditions  is  essential  to  the  value  of  a 
rock  analysis:  the  specimen  analyzed  must  be  representative  of 
the  rock-mass,  and  the  analysis  itself  must  truly  represent  the 
composition  of  the  specimen  selected.  The  more  closely  both  of 
these  conditions  are  met,  the  greater  will  be  the  value  of  the 
analysis. 

The  representative  character  of  the  specimen  is  determined 
by  the  character  of  the  rock-mass,  as  influencing  both  its  selection 
and  the  amount  of  material  taken  for  analysis.  These  points 
will  be  discussed  subsequently  (p.  58). 

Assuming  that  the  sample  is  representative  of  the  rock- 
mass,  the  degree  of  correspondence  between  the  figures  yielded 
by  the  analysis  and  the  real  chemical  composition  of  the  rock 
is  dependent  on  the  two  factors  of  accuracy  and  completeness. 

Accuracy  of  Analyses. — By  accuracy  is  meant  the  degree  of 
precision  with  which  the  constituents  sought  for  are  determined, 
quite  apart  from  whether  or  not  all  of  those  present  have  been 
determined  or  separated  from  one  another.  The  accuracy  of  an 
analysis  is  dependent  upon  the  methods  used  and  upon  the  ability 
of  the  analyst  to  execute  the  various  processes  successfully.  The 
purity  of  the  reagents  and  the  adequacy  of  the  apparatus  are 
also  factors. 

1  This  and  the  next  two  sections  are  a  somewhat  summarized  statement  of 
part  of  the  discussion  published  in  Prof.  Paper  U.  S.  Geological  Survey,  99, 
pp.  10-18,  1917. 


4  INTRODUCTION 

It  must  be  borne  in  mind  that  no  method  is  capable  of  yielding 
results  of  absolute  accuracy,  any  more  than  it  is  possible  to  con- 
struct a  mathematically  exact  geometrical  figure.  Certain 
sources  of  error  are  inherent  in  all,  some  of  a  general  nature,  and 
others  of  a  character  dependent  upon  the  method  employed. 
The  analyst  must  rest  content  with  reducing  these  to  a  minimum, 
by  selecting  methods  which  have  been  shown  to  be  reliable.  In 
this  we  cannot  do  better  than  follow  the  chemists  of  the  U.  S. 
Geological  Survey,  whose  experience  is  of  the  widest,  and  who  have 
set  up  a  standard  of  analytical  methods  and  practice  for  rocks  and 
minerals  that  is  beyond  all  others. 

But  the  selection  of  proper  methods  is  not  the  only  desidera- 
tum. They  must  be  carried  out  in  a  proper  way,  which  will 
not  lead  to  errors  of  a  purely  mechanical  kind  that  may  easily 
vitiate  the  results  of  the  theoretically  most  accurate  method.  In 
this  matter  the  analyst  himself  is  the  most  important  factor. 
He  should  have,  not  only  sufficient  knowledge  of  the  facts  of  chem- 
istry and  of  the  principles  of  analysis  to  work  understandingly, 
but  also  the  dexterity  and  manipulative  skill  to  enable  him  to 
carry  out  the  various  processes  successfully.  While  it  may  be 
true  of  some  analysts  that,  like  poets,  they  are  born,  not  made,  yet 
granted  intelligence  and  chemical  knowledge  and  a  fair  amount 
of  dexterity  and  application,  the  necessary  manipulative  skill  will 
come  with  practice,  often  in  a  surprisingly  short  time. 

The  analyst  should  beware  of  falling  into  careless  habits 
or  of  allowing  the  analysis  to  become  merely  routine  work.  Care- 
lessness is  as  fatal  to  obtaining  good  results  as  poor  methods  or 
impure  reagents.  During  the  whole  progress  of  an  analysis 
attention  should  be  paid  to  every  point  of  theory  or  manipulation, 
the  influence  of  the  various  conditions  or  constituents  should  be 
considered,  and  indeed  the  analysis  should  be  carried  out  from 
beginning  to  end  with  intelligent  interest.  This  will  turn  into  a 
pleasure  what  would  otherwise  be  a  dull  and  monotonous  suc- 
cession of  precipitations,  filtrations,  ignitions  and  weighings, 
which,  as  has  been  justly  said,  is  not  chemical  analysis. 

That  conscientiousness,  a  strict  regard  for  the  truth,  and  a 
firm  determination  to  accept  no  result  of  doubtful  character,  are 
essential  to  the  analyst  goes  without  saying. 

It  may  be  said  that  the  analysis  of  rocks  (and  minerals)  would 


GENERAL  CHARACTER  OF  ANALYSES          5 

seem  to  be  especially  suitable  for  women ;  whose  characteristics  of 
neatness,  patience,  application,  care  and  conscientiousness,  and 
attention  to  detail,  would  be  most  valuable  in  analytical  work.  I 
know  of  but  three  women  who  have  made  any  considerable  number 
of  rock  analyses,  and  have  found  the  work  of  all  to  be  uniformly 
good.1 

Completeness  of  Analyses. — As  regards  completeness,  the  ideal 
analysis  should  show  the  percentage  of  every  constituent  in  the 
rock  as  well  as  the  absence  of  such  whose  presence  might  have 
been  expected  on  the  basis  of  previous  experience.  This  is  not 
always  attainable,  and  for  practical  purposes  the  analysis  should 
give  figures  for  all  constituents  which  are  present  in  sufficient 
amount  to  make  their  determination  a  matter  of  interest,  or 
whose  presence  or  absence  may  bear  on  the  problem  for  which  the 
analysis  is  made. 

The  number  of  constituents  which  should  be  sought  for  and 
determined  depends,  of  course,  very  largely  on  the  character  of 
the  rock.  Thus,  in  most  granites,  quartz-porphyries  and  rhyolites 
which  are  of  simple  composition,  comparatively  few  constituents 
need  be  determined  to  make  the  analysis  satisfactory.  On  the 
other  hand,  in  such  rocks  as  nephelite-syenites,  diorites,  basalts, 
and  tephrites,  the  number  of  constituents  which  should  be  deter- 
mined is  larger,  and  may  easily  reach  twenty  or  more. 

It  is  to  be  borne  in  mind  that  neglect  to  seek  for  some  of 
the  rarer  constituents  may  lead  to  the  overlooking  of  important 
features,  and  that  an  analysis  complete  as  to  the  subsidiary  con- 
stituents may  be  of  great  value  in  the  future,  even  if  this  degree 
of  completeness  is  not  necessary  for  the  end  immediately  in  view. 
The  analyst  should  turn  out,  and  the  petrologist  should  be  willing 
to  accept,  only  results  of  the  highest  character;  so  that  it  follows, 
as  a  general  thing,  that  every  analysis  should  be  as  complete  as  it 
is  possible  to  make  it. 

The  details  of  the  constituents  to  be  determined  will  be  taken 
up  later  (p.  7),  but  it  may  be  stated  here  in  a  general  way  that  all 
the  main  constituents  must  be  determined  in  every  analysis,  as 
well  as  those  minor  ones  which  enter  into  the  composition  of  min- 
erals that  are  present  in  notable  amount.  If  the  general  character 
of  the  petrographical  province  or  the  microscopical  examina- 
1  Cf.  Washington,  Prof.  Paper,  99,  p.  23. 


6  INTRODUCTION 

tion  indicates  the  presence  of  certain  of  the  rarer  elements,  these 
should  also  be  looked  for. 

3.  MICROSCOPICAL  EXAMINATION 

The  chemical  analysis  should  always  be  preceded  by  a  micro- 
scopical examination  by  the  petrographer  of  the  rock  in  thin 
section.  There  are  several  reasons  for  this.  In  the  first  place,  by 
a  comparison  of  several  specimens  in  thin  section  one  is  able  to 
judge,  better  than  by  a  merely  megascopic  examination,  whether 
the  specimen  selected  for  analysis  may  be  considered  as  really  a 
representative  one.  It  has  happened  more  than  once  that  speci- 
mens selected  for  analysis  without  such  microscopic  study  have 
been  shown  later  to  be  abnormal  forms  and  not  typical  of  the  rock 
mass  or  volcano  under  investigation;  or  else  that  several  analyses 
have  been  made  of  one  kind  of  rock,  while  equally  important  kinds 
have  not  been  analyzed. 

The  microscope  also  frequently  gives  important  indications 
as  to  the  presence  of  rare  constituents  which  should  be  determined, 
or  the  absence  of  others  which  may  therefore  be  neglected.  It 
will  thus  often  prevent  the  overlooking  of  constituents  the  deter- 
mination of  which  may  be  of  considerable  importance,  or,  on  the 
other  hand,  may  save  much  labor  and  time  to  the  analyst  in  search- 
ing for  substances  which  are  not  present,  at  least  in  determinable 
amount. 

Thus,  if  microscopic  zircons  are  present  in  a  granite,  the 
amount  of  zirconia  should  be  determined  to  render  the  analysis 
satisfactorily  complete,  while  if  these  are  absent  this  substance 
can  be  neglected  without  serious  diminution  in  the  value  of  the 
analysis.  The  presence  of  crystals  of  a  colorless,  isotropic  mineral, 
of  low  refractive  index,  will  necessitate  the  determination  of  Cl 
and  SO3,  as  they  may  be  crystals  of  colorless  sodalite  or  haiiyne, 
while  if  none  are  found  under  the  microscope  in  a  holocrystalline 
rock  these  constituents  may  usually  be  considered  as  absent. 

Finally,  the  thin  section  will  show  much  more  definitely  than 
the  hand  specimen  whether  the  rock  is  fresh  and  unaltered  enough 
to  justify  its  analysis. 

It  should  also  be  noted  that  the  percentage  amount  of  certain 
constituents  may  sometimes  be  determined  by  the  microscope 
with  almost  as  much  accuracy  as  by  chemical  analysis,  and  often 


CONSTITUENTS  TO   BE  DETERMINED  7 

with  greater  ease  and  expedition.  This  will  be  true  for  some  which 
are  present  only  in  very  small  amounts  and  which  occur  in  minerals 
of  definite  composition. 

Thus,  if  zirconia  is  present  only  in  zircon,  or  fluorine  in  fluorite, 
or  sulphur  in  pyrite,  the  amount  of  these  minerals  in  the  rock  can 
be  readily  estimated  by  Rosiwal's  method,1  and  the  percentage 
of  Zr02,  F,  or  S,  respectively,  may  be  easily  calculated.  Though 
this  method  also  applies  to  phosphorus  pentoxlde  in  apatite,  yet 
this  substance  is  of  such  importance  as  a  minor  constituent,  and 
its  determination  analytically  is  so  easy  and  expeditious,  that  its 
amount  should  always  be  ascertained  in  the  regular  analytical  way. 
Except  possibly  for  fluorine  existing  only  in  fluorite,  this  micro- 
scopical method  is,  however,  less  satisfactory  than  the  chemical, 
and  if  it  is  adopted,  a  note  to  that  effect  should  be  made  in  the 
statement  of  the  analysis. 

4.  CONSTITUENTS  TO  BE  DETERMINED 

Importance  of  Completeness. — In  the  earlier  days  of  petrog- 
raphy, the  petrographer  was  quite  content  if  the  analyst  reported 
figures  for  only  eight  or  nine  constituents,  and  he  did  not  always 
insist  on  the  separation  of  the  two  oxides  of  iron.  One  seldom 
meets  with  analyses  of  this  period  in  which  TiO2  or  ?2O5  are 
mentioned,  to  say  nothing  of  such  substances  as  ZrC>2,  BaO  or  F. 
In  the  absence  of  exact  knowledge  of  the  mineral  composition  of 
rocks  the  presence  of  such  rare  elements  was  not  often  suspected, 
nor  did  neglect  of  them  in  the  course  of  the  analysis  necessarily 
cause  such  low  summations  as  to  give  rise  to  suspicions  that 
something  had  been  overlooked.  This  was  partly  because  these 
rarer  elements  almost  invariably  occur  in  very  small  amounts, 
partly  because  some  of  them,  as  TiO2,  P2O5,  Zr(>2,  CfoOs  and  SrO, 
are  precipitated  and  weighed  with  other  constituents,  and  partly 
because  the  analyst  of  those  days  was  not  as  accurate  in  his  methods 
as  at  present,  and  was  content  with  a  summation  which  would 
cause  the  rejection  or  the  doing  over  of  the  analysis  by  a  modern 
chemist. 

After  it  became  possible  to  study  rocks  in  thin  section,  and  when 

1  Rosiwal,  Verh.  Wien.  Geol.  Reichs-Anst.,  32,  p.  143,  1898.  Cf.  Cross, 
Iddings,  Pirsson,  and  Washington,  Quant.  Class.  Igneous  Rocks,  Chicago, 
1903,  p.  204. 


8  INTRODUCTION 

the  use  of  heavy  solutions  made  the  separation  of  the  component 
minerals  easy,  it  was  found  that  the  number  of  chemical  constit- 
uents commonly  present  in  rocks  was  far  larger  than  had  been 
supposed,  although  the  importance  of  determining  them  was  not 
recognized  for  many  years.  With  improvement  in  old  methods 
and  the  adoption  of  new  ones,  the  determination  of  these  minor 
constituents  was  greatly  facilitated,  and  at  the  present  day 
analyses  in  which  figures  are  reported  for  twenty  or  more  con- 
stituents are  common,  at  least  in  some  countries;  though,  unfor- 
tunately, there  is  still  a  tendency  among  many  chemists  to  rest 
content  with  the  determination  of  only  the  more  notable  in- 
gredients. 

At  first  sight  it  may  not  seem  worth  while  to  pay  attention 
to  constituents  which  are  present  in  amounts  only  up  to  a  few 
tenths  of  a  per  cent.  But  there  are  very  good  reasons  for  not 
neglecting  them. 

For  one  thing  the  determination  or  non-determination  of  some 
of  them  affects,  and  may  affect  seriously,  the  figures  for  other  and 
more  important  constituents.  This  is  because  several  of  them  are 
precipitated  and  weighed  together,  and  then  all  except  one  are 
determined  separately,  so  that  the  figure  for  the  final  one  depends 
on  those  of  the  others,  since  it  is  determined  by  difference. 

Thus,  A12O3,  Fe2O3,  Cr2O3,  V203,  TiO2,  Zr02,  P205,  MnO, 
and  a  little  SiO2  are  thrown  down  and  weighed  together,  all  except 
the  first  are  determined  separately,  and  the  weight  of  the  A12O3, 
which  is  usually  greater  than  all  the  others  combined,  is  ascer- 
tained from  the  difference.  It  is  evident  that  if  any  one  of  the 
other  oxides  is  neglected  the  figure  for  alumina  will  be  too  high, 
and  this  error  may  be  serious.  Similar  cases  are  those  of  CaO 
and  SrO  and  of  P2Os  and  V2O5,  though  in  these  the  error 
involved  will  seldom  be  of  great  moment. 

Again,  the  non-determination  of  such  constituents  as  are 
not  precipitated  and  weighed  with  others,  will  lower  the  summation 
of  the  analysis.  This  lowering  may  amount  to  so  much  as  to  cause 
an  otherwise  superior  analysis  to  appear  to  be  inferior.1  Such 
non-determinations  are  those  of  H2O,  C02,  SO3,  Cl,  S,  NiO,  and, 
in  some  cases,  MnO. 

Another,  and  equally  important  reason  for  completeness  is 
1  Cf.  Washington,  Prof.  Paper,  99,  p.  31. 


CONSTITUENTS  TO  BE  DETERMINED  9 

that  evidence  is  accumulating,  as  analyses  of  a  high  degree  of 
completeness  become  more  common,  that  much  light  may  be 
thrown  upon  petrological  problems  of  great  interest  by  a  knowl- 
edge of  the  presence  of  the  rare  elements.  The  subject  has  been 
discussed  by  Hillebrand,1  whose  strong  plea  for  completeness  it 
will  be  well  for  the  student  to  read.  An  illustration  given  by 
Hillebrand  may  be  cited  here.  The  analyses  of  the  U.  S.  Geolog- 
ical Survey  show  that  baryta  and  strontia  are  almost  invariably 
present  in  the  igneous  rocks  of  the  United  States,  and  that  the 
former  is  uniformly  in  greater  amount  than  the  latter.  Further- 
more it  is  made  clear  that,  while  never  present  in  large  amount, 
they  are  both  more  abundant  in  the  rocks  of  the  Rocky  Mountain 
region  than  in  those  to  the  east  and  west  of  this.  As  Hillebrand 
says:  "  Surely  this  concentration  of  certain  chemical  ele- 
ments in  certain  geographical  zones  has  a  significance  which 
future  geologists  will  be  able  to  interpret,  if  those  of  to-day  are 
not." 

Similarly,  we  now  know,  through  the  complete  analyses  inau- 
gurated by  the  chemists  of  the  U.  S.  Geological  Survey,  that 
titanium  is  not  the  rare  element  it  was  formerly  believed  to  be, 
but  is  ninth  in  order  of  abundance  of  the  elements  that  make  up 
the  known  crust  of  the  earth,  being  present  to  the  amount  of 
about  one-half  of  one  per  cent.2 

Another  interesting  result  of  the  determination  of  the  rarer 
elements  is  the  discovery  that  certain  of  them  are  associated 
more  especially  with  magmas  of  certain  characters,  but  are  seldom 
found  in  rocks  derived  from  magmas  of  other  chemical  types.3 
Thus,  it  has  been  shown  by  Hillebrand  4  that  vanadium  is  most 
abundant  in  rocks  that  are  low  in  silica,  while  it  is  absent,  or  nearly 
so  in  the  highly  siliceous  rocks;  conversely  molybdenum  is  con- 
fined apparently  to  the  highly  siliceous  rocks.4  It  is  now  well- 
known  that  high  percentages  of  zirconium  and  of  the  rare  earths 
are  most  frequent  in  highly  sodic  rocks,  while  notable  amounts  of 

1  W.  F.  Hillebrand,  Jour.  Am.  Chem.  Soc.,  16,  p.  90,  1894;  Bull.  422,  p.  16; 
H.  S.  Washington,  Prof.  Paper,  99,  p.  16. 

2  Cf.  F.  W.  Clarke,  The  Data  of  Geochemistry.     U.  S.  Geological  Survey 
Bulletin  616,  pp.  27,  34,  1916. 

3  For  a  discussion  of  this  subject  and  references  see  Washington,  Trans. 
Am.  Inst.  Min.  Eng.,  30,  p.  735,  1908. 

4  Hillebrand,  pp.  20,  21,  148. 


10  INTRODUCTION 

chromium  or  nickel  are  seldom  met  with  in  rocks  that  are  not  high 
in  magnesia  and  iron  and  low  in  silica  and  alkalies. 

This  leads  directly  to  the  consideration  of  a  final  point  in 
favor  of  the  present  contention,  namely,  the  light  that  may  be 
thrown  on  the  origin  and  formation  of  ores,  and  the  possibility  of 
such  chemical  study  of  the  igneous  and  metamorphic  rocks  lead- 
ing in  the  future  to  important  economic  advances  in  the  indi- 
cation of  the  presence  of  ore  bodies.  The  researches  of  Sand- 
berger  and  others  l  have  shown  that  many  of  the  heavy  metals, 
such  as  antimony,  arsenic,  bismuth,  cobalt,  copper,  lead,  silver, 
tin,  uranium,  and  zinc,  are  present  in  the  pyroxenes,  hornblendes, 
biotites  and  olivines  of  some  igneous  rocks,  and  can  be  readily 
detected  if  sufficiently  large  amounts  are  taken  for  analysis. 
Further  consideration  of  this  topic  is  uncalled  for  here,  but,  from 
the  point  of  view  of  the  mining  engineer  and  of  geological  surveys, 
it  is  clear  that  this  is  a  weighty  argument  in  favor  of  completeness 
and  the  determination  of  minor  constituents  in  the  making  of 
chemical  analyses  of  rocks. 

While  it  follows  from  the  above  that  all  rock  analyses  should 
ideally  be  as  complete  as  it  is  possible  to  make  them,  yet  the  prac- 
tical considerations  of  time  and  labor  may  set  limitations  on  this. 
Although  by  judicious  management  a  number  of  the  minor  con- 
stituents can  be  determined  along  with  the  main  ones,  and  at 
the  cost  of  very  little  extra  time,  yet  it  is  true  that  a  thoroughly 
complete  analysis  will  take  considerably  longer  than  a  simple 
one.  The  analyst  must  judge  for  himself  how  far  he  can  profit- 
ably go  in  this  way,  but  it  should  be  borne  in  mind  that  a  few 
complete  analyses  will  probably  be  of  more  value  in  the  end  than  a 
larger  number  of  incomplete  ones.2 

i  While  it  is  probable  that  all  or  nearly  all  of  the  known  elements 
may  occasionally  be  present  in  rocks,  and  can  be  detected  if  suf- 
ficiently large  amounts  are  taken  for  analysis,  in  practice  we  must, 
for  the  purposes  of  this  volume,  confine  our  attention  to  those  which 
may  reasonably  be  looked  for  in  igneous,  metamorphic,  and  many 
silicate  sedimentary  rocks,  and  which  may  be  readily  estimated  in 
quantities  of  from  one-half  to  two  grams  of  material.  Those 

1  F.  Sandberger,  Zeits.  Deutsch.  Geol.  Ges.  32,  p.  350,  1880;  Zeits.  Prakt. 
Geol.,  1896;  cf.  J.  H.  L.  Vogt,  Zeits.  Prakt.  Geol.,  1898,  pp.  225  ff. 

2  See  page  16  for  a  suggestion  as  to  a  practical  procedure  in  regard  to  this. 


CONSTITUENTS  TO  BE  DETERMINED  11 

which  will  be  considered  in  this  book  are  given  in  the  following  list, 
which  is  substantially  that  of  Hillebrand: 

SiO2,  TiO2,  ZrO2,  A12O3,  Fe2O3,  Cr2O3,  V2O3,  B2O3,  (Ce,  Y)203, 
FeO,  MnO,  NiO,  CoO,  MgO,  CaO,  SrO,  BaO,  CuO,  Na2O,  K2O, 
Li2O,  H2O,  CO2,  P2O5,  Cl,  F,  SO3,  S. 

In  addition,  such  elements  of  rare  occurrence  in  igneous  rocks 
as  carbon  (as  graphite  or  organic  matter),  glucinum,  lead,  molyb- 
denum, nitrogen,  tin,  or  zinc,  may  be  present  in  determinable 
amount,  but  these  occurrences  are  so  seldom  met  with,  and  the 
determination  of  such  constituents  involves  so  complicated 
methods  that  they  will  not  be  considered  in  this  book. 

In  the  great  majority  of  rocks  the  constituents  of  the  list  just 
given  are  by  no  means  of  equal  importance,  and  it  is  customary 
to  divide  them  into  "  main  "  and  "  minor  "  constituents. 

Main  Constituents. — Speaking  generally,  the  main  constit- 
uents are  Si02,  A12O3,  Fe203,  FeO,  MgO,  CaO,  Na2O,  K2O, 
H2O. 

These  nine  (including  both  oxides  of  iron)  are  almost  invari- 
ably present  in  greater  or  less  amount  in  all  igneous  and  meta- 
morphic  silicate  rocks,  and  positively  must  be  determined  in 
every  rock  analysis  if  it  is  to  conform  to  even  the  first  require- 
ment as  to  completeness. 

The  only  possible  exceptions  would  be  certain  rare  and  little- 
known  types,  with  which  the  student  is  not  likely  to  meet.  Thus, 
in  iron  ores  produced  by  differentiation  of  an  igneous  magma,  or 
in  dunites,  the  amount  of  alkalies  may  be  so  small  as  to  be  negli- 
gible for  most  purposes.  Or,  in  the  case  of  very  highly  quartzose 
dikes  of  igneous  origin,  such  as  have  been  described  from  Africa 
and  Australia,  the  determination  of  lime  and  especially  magnesia 
may  be  omitted.  But  even  in  such  rocks  it  is  far  better  to  prove 
definitely  that  these  constituents  are  absent  or  present,  even  if 
only  in  traces.  In  the  light  of  physico-chemical  investigations  of 
extremely  dilute  solutions,  this  knowledge  may  be  of  great  interest 
and  importance  in  the  future. 

Stress  must  be  laid  on  the  importance  of  the  separate  deter- 
mination of  both  oxides  of  iron,  which  have  been  only  too  often 
unseparated  and  reported  in  the  analysis  as  either  Fe2O3  or  FeO. 
Neglect  of  this  point  was  especially  common  up  to  thirty  years 
ago,  and  is  the  cause  of  the  relative  worthlessness  of  many  of  the 


12  INTRODUCTION 

older  analyses.1  It  is  clear  that,  as  the  two  oxides  play  different 
roles  in  the  composition  of  minerals,  a  knowledge  of  the  relative 
amounts  of  each  is  necessary  to  a  proper  understanding  of  the  rock 
magma,  the  calculation  of  the  mode  (actual  mineral  composition) 
of  the  rock,  or  for  its  classification  along  chemico-mineralogical 
lines.  Although  the  error  involved  by  their  non-separation  may 
be  small  in  certain  highly  quartzose  or  feldspathic  rocks,  as 
granites,  rhyolites,  trachytes  and  syenites,  in  which  they  do  not 
amount  together  to  more  than  one  or  two  per  cent,  yet  the  con- 
scientious analyst  should  always  make  it  a  point  to  determine 
them  separately. 

Water  is  often  present  in  very  notable  amount,  and  should 
therefore  be  reported  in  every  rock  analysis,  even  though  it  is  true 
that  the  amount  of  water  is  not  vital  to  our  knowledge  of  the 
rock  magma,  except,  of  course,  when  minerals  containing  water  of 
crystallization  or  hydroxyl,  such  as  analcite  and  muscovite,  are 
present;  yet  its  determination  is  important  in  that  it  gives  a 
measure  of  the  freshness  of  the  rock.  There  is  all  the  more  reason 
for  doing  this  both  on  account  of  the  ease  and  celerity  of  the 
determination  and  on  account  of  the  fact  that  its  neglect  may 
seriously  affect  the  summation  of  the  analysis.  It  is  also  evident 
that  the  determination  is  essential  in  the  investigation  of  many 
metamorphic  and  sedimentary  rocks,  and  in  the  study  of  rock 
weathering  and  alteration,  where  hydrous  minerals,  as  chlorite, 
zeolites,  and  limonite,  are  present. 

Water  may  be  present  as  either  or  both  "  hygroscopic  "  and 
"  combined  "  water,  which  are  expelled  from  the  rock  powder  at 
temperatures  respectively  below  and  above  about  110°,  the  con- 
ventional temperature.  There  is  considerable  difference  of  opin- 
ion as  to  the  advisability  of  the  separate  determination  of  these, 
as  well  as  to  the  reporting  of  the  hygroscopic  water  in  the  analysis. 
The  arguments  for  and  against  their  separation  have  been  dis- 
cussed by  Hillebrand,2  and  need  not  be  repeated  here.  While  the 
matter  is  not  of  great  importance,  it  may  suffice  to  say  that  I 
concur  with  the  opinion  of  Hillebrand  in  recommending  their 
separate  determination  and  inclusion  in  the  statement  of  the 
analysis,  and  also  in  the  use  of  air-dried  material  for  analysis. 

1  Cf.  Washington,  Prof.  Paper,  99,  pp.  15,  21,  26. 

2  Hillebrand,  pp.  57-63. 


CONSTITUENTS  TO  BE  DETERMINED  13 

Apart  from  the  constituents  discussed  above,  there  are  a  num- 
ber of  usually  minor  ones,  which  may  in  some  rocks  assume  equal 
importance  with,  or  even  far  surpass,  some  of  the  main  constituents 
enumerated  above.  While  examples  of  this  are  uncommon,  yet 
their  number  is  rapidly  growing  with  increase  in  our  knowledge 
of  the  less  well-known  rocks  of  the  globe,  and  many  of  them  are  of 
special  interest  from  the  theoretical  side.  As  examples  there  may 
be  cited  titaniferous  ores  produced  by  differentiation,  as  those  of 
the  Adirondacks,  the  apatite — rich  nelsonites  of  Virginia,  such 
sodalite-  and  hauyne-rich  rocks,  as  tawite,  taimyrite,  and  haiiyno- 
phyres,  the  eudialyte-rich  lujavrites  of  Kola  and  Greenland,  or  the 
apparently  igneous  pyritiferous  ores  of  Norway.  In  these,  cer- 
tain constituents  which  are  usually  regarded  as  minor,  Ti(>2, 
P2Os,  Cl,  SOs,  Zr(>2  and  S,  respectively,  are  of  an  importance 
equal  to  that  of  any  of  the  nine  mentioned  above,  and  it  is  clear 
that  an  analysis  of  such  rocks  which  does  not  take  these  usually 
minor  constituents  into  account  is  seriously  defective. 

Minor  Constituents. — The  minor  constituents  differ  much  in 
their  relations  to  the  analysis.  Those  of  one  group  are  precip- 
itated and  weighed  with  some  of  the  main  constituents  (as  has 
been  mentioned  above),  and  their  weight  is  afterward  to  be  sub- 
tracted from  that  of  the  mixed  precipitate.  Therefore,  if  they  are 
not  determined  the  apparent  amount  of  the  main  constituent, 
which  is  determined  by  difference,  will  be  too  great.  This  is  true, 
for  instance,  with  the  oxides  of  titanium,  zirconium,  chromium, 
vanadium,  and  phosphorus,  and  the  rare  earths.  These  are  all 
precipitated  and  weighed  with  the  alumina,  and  if  any  one  of  them 
is  disregarded  it  will  increase  by  its  weight  the  apparent  amount  of 
the  alumina.  The  resultant  error  may  not  be  large  but,  being 
avoidable,  should  not  be  committed  by  the  careful  analyst. 

Of  these  minor  constituents,  the  most  important  are  titanium 
dioxide  and  phosphorus  pentoxide,  which  are  almost  invariably 
present  and  in  most  rocks  in  quantities  so  large  as  to  cause  serious 
error  in  the  figure  for  alumina  if  their  determination  is  neglected. 
These  two  should,  on  this  account,  be  determined  in  every  rock 
analysis,  or  its  value  may  be  notably  diminished,  because  knowl- 
edge of  the  exact  amount  of  alumina  is  a  very  important  factor  in 
chemico-mineralogical  rock  classifications,  as  well  as  in  the  calcu- 
lation of  the  mineral  composition.  The  others  are  seldom  present 


14  INTRODUCTION 

in  amount  greater  than  a  few  tenths  of  one  per  cent  and  usually 
much  less,  so  that  neglect  of  them  will  not  often  involve  appre- 
ciable error  in  the  figure  for  alumina.  Zirconia  is  apt  to  occur  in 
notable,  though  small,  amounts  in  rocks  that  are  high  in  soda,  so 
that  it  is  well  to  determine  zirconia  in  such  rocks,  while  chromium 
sesquioxide  and  vanadium  pentoxide  1  may  be  determined  in  rocks 
that  are  high  in  magnesia  and  iron  and  low  in  silica. 

Analogous  cases  are  strontia,  lithia,  and  manganous  oxide. 
The  first  of  these  is  precipitated  and  weighed  with  lime,  being 
afterward  separated  from  this  to  arrive  at  the  true  weight  of  the 
lime.  Similarly,  lithia  is  weighed  with  soda,  thus  increasing  its 
apparent  amount.  But  both  strontia  and  lithia  (especially  the 
latter)  are  present  in  such  minute  amounts  that  their  non-deter- 
mination will  seldom  affect  the  figures  for  lime  or  soda  to  an  appre- 
ciable extent.  They  are  chiefly  of  interest  from  the  theoretical 
side,  and  this  applies  more  especially  to  strontia. 

The  case  of  manganous  oxide  is  more  complex,  and  for  its  dis- 
cussion we  must  anticipate  some  features  of  the  analysis.  If  the 
alumina  is  precipitated  with  ammonia  water,  as  is  usually  done 
some  of  the  manganese  comes  down  with  the  alumina,  a  little 
of  it  comes  down  later  with  the  lime,  and  the  rest  falls  with  the 
magnesia.2  It  is  clear,  therefore,  that  unless  the  manganous  oxide 
is  completely  separated  from  the  alumina,  and  if  it  is  not  precip- 
itated before  the  determination  of  lime  and  magnesia,  it  will  be 
distributed  among  these  three  constituents. 

On  the  other  hand,  whi  e  it  may  be  completely  separated  from 
the  alumina  by  what  is  known  as  the  "  basic  acetate  "  precipita- 
tion, and  then  precipitated  as  sulphide  before  lime  and  magnesia 
are  determined,  yet  this  procedure  is  liable,  as  we  shall  see 
pp.  (149,  155),  to  cause  serious  errors  in  the  determination  of 
alumina  and  iron  oxide;  likewise  it  is  tedious  and  involves  about 
one  day's  extra  time. 

Fortunately  manganous  oxide  is  present  in  very  few  rocks  in 
greater  amounts  than  two  or  three-tenths  of  one  per  cent,  so  that 

1  Vanadium   pentoxide   is  also   later  precipitated   and  weighed  with   the 
phosphorus  pentoxide,  so  that  its  amount  should  be  subtracted  from  that  of 
the  latter,  not  from  that  of  the  alumina.     See  page  216. 

2  This  distribution  of  manganese  has  been  studied  by  G.  Steiger,  in  the 
U.  S.  Geological  Survey  Laboratory.     Cf.  Hillebrand,  p.  114;   Mellor,  p.  371. 


CONSTITUENTS  TO  BE  DETERMINED  15 

even  if  it  is  neglected  and  its  weight  distributed  among  alumina, 
lime,  and  magnesia,  the  error  will  not  usually  be  serious,  and  may 
be  regarded  as  negligible. 

Manganous  oxide  can,  however,  be  thrown  down  with  the 
alumina  by  ammonium  persulphate,  so  that  it  will  not  subse- 
quently affect  the  figures  for  lime  and  magnesia.  It  can  be  de- 
termined separately  by  a  colorimetric  method,  and  its  weight  can 
then  be  subtracted  from  that  of  alumina.  This  is  the  best  pro- 
cedure in  the  great  majority  of  rock  analyses,  and  we  can  say  that 
manganous  oxide  should  always  be  determined  colorimetrically, 
although  it  need  not  be  separated  from  the  other  more  important 
constituents  if  its  amount  is  very  small. 

The  second  group  of  minor  constituents  consists  of  those  whose 
determination  or  non-determination  does  not  affect  the  figures  for 
any  of  the  main  constituents.  This  includes  nickel,  cobalt,  copper, 
and  barium  oxides,  sulphur,  sulphur  trioxide,  chlorine,  fluorine, 
and  carbon  dioxide. 

Of  these  the  first  three  occur  as  a  rule  only  in  traces  in  igneous 
rocks,  nickel  more  abundantly  in  the  less  siliceous  ones;  in  these 
it  may  be  well  to  determine  it.  Indeed,  the  determination  of 
nickel  is  advisable  in  very  particular  analyses  of  intermediate 
to  basic  rocks,  and  is  always  necessary  in  analyzing  meteorites, 
though  neglect  of  it  will  seldom  if  ever  lead  to  serious  error  in 
dealing  with  terrestrial  rocks.  Copper  cannot  be  considered  an 
important  constituent,  but  it  may  well  be  looked  for  in  rocks  low 
in  silica,  as  it  may  be  of  theoretical  interest. 

As  has  been  mentioned  above,  barium  is  a  constant  con- 
stituent of  the  igneous  rocks  of  the  United  States,  and  it  is  almost 
certain  that  it  will  be  found  to  be  widely  distributed  elsewhere 
when  it  is  looked  for  systematically.  In  view  of  its  theoretical 
interest  and  the  comparative  ease  of  its  determination  by  the 
method  given  beyond,  it  will. usually  be  advisable  to  look  for  it  in 
the  course  of  the  analysis. 

Sulphur  is  very  frequently  present  as  the  sulphides  pyrite 
and  pyrrhotite,  and  indeed  much  more  often  than  was  formerly 
believed,  especially  in  the  less  siliceous  rocks.  Its  amount  can 
be  readily  ascertained  along  with  the  baryta  and  it  should  enter 
into  the  statement  of  most  analyses,  or  its  absence  be  definitely 
shown. 


16  INTRODUCTION 

Sulphur  trioxide  and  chlorine  are  met  with  in  igneous  rocks 
with  comparative  frequency,  especially  in  rocks  that  are  high  in 
soda,  and  both  are  always  to  be  determined  if  minerals  of  the 
sodalite  group  are  present.  It  is  always  well  to  determine  them  in 
rocks  liable  to  carry  such  minerals,  even  if  these  are  not  visible 
with  the  microscope.  In  other  rocks  also  it  can  scarcely  be  held 
to  be  a  great  loss  of  time  to  look  for  them,  in  view  of  their  possible 
theoretical  interest  and  the  ease  of  their  determination. 

Fluorine  is  seldom  present  in  quantities  over  a  few  tenths 
of  a  per  cent  and  need  not  often  be  looked  for.  This  may  be 
done,  however,  if  the  rock  contains  fluorine-bearing  minerals,  but 
even  here  its  determination  is  necessary  only  if  these  are  abundant 
or  for  very  accurate  work. 

Carbon  dioxide  is  often  present  in  igneous  and  metamorphic 
silicate  rocks,  but,  as  far  as  is  now  known  with  certainty,  only 
when  the  rock  is  not  strictly  fresh,  as  a  component  of  the  secondary 
minerals,  calcite,  dolomite,  siderite  and  cancrinite.  If  it  is  present 
it  should  always  be  determined,  because  it  serves  to  a  certain 
extent  as  a  measure  of  the  freshness  of  the  rock,  and  because  the 
result  may  have  a  bearing  on  the  problem  of  its  occurrence  as  a 
primary  constituent. 

It  is  to  be  remembered  that  if  water  is  determined  as  "  loss  on 
ignition,"  this  will  include  the  carbon  dioxide,  which  is  also  ex- 
pelled. The  latter  must,  therefore,  be  determined  separately  and 
directly. 

As  a  final  and  practical  solution  of  the  difficulty  in  deciding 
what  constituents  to  determine  the  following  procedure  is  sug- 
gested.1 If  only  one  or  two  rocks  are  to  be  analyzed,  especially 
if  they  come  from  a  little-known  locality,  the  analysis  should  be  as 
complete  as  time  and  other  considerations  will  permit.  But  if  a 
series  of  analyses  is  to  be  made  of  the  rocks  of  a  region  or  volcano 
that  includes  various  types,  two  analyses  should  be  made  of  each  of 
the  most  important  or  most  abundant  types,  and  but  one  analysis 
of  a  representative  of  each  of  the  less  important.  All  of  these 
analyses  should  show  the  amounts  of  the  main  constituents,  as 
well  as  the  minor  ones,  TiO2,  ?2O5,  and  MnO,  including  the  usually 
minor  ones  which  may  be  of  major  importance.  The  rarer  con- 
stituents, such  as  ZrO2,  CfoOs,  BaO,  SrO,  and  NiO,  need  only  be 
1  Cf.  Washington,  Prof.  Paper,  99,  p.  17. 


THE  OCCURRENCE  OF  VARIOUS  ELEMENTS      17 

determined  in  one  each  of  the  more  important  types.  This  may  be 
regarded  as  the  optimum  compromise  between  the  ideal  and  the 
practical. 

5.  THE  OCCURRENCE  OF  VARIOUS  ELEMENTS 

The  increased  number  of  analyses  of  igneous  rocks,  especially 
of  those  of  unusual  types,  and  the  more  frequent  determination 
of  the  minor  constituents,  with  the  mass  of  data  obtained  by 
the  use  of  the  microscope,  have  shown  that  certain  of  the  rarer 
elements  are  prone  to  occur  in  rocks  of  certain  chemical  characters. 
While  our  knowledge  along  this  line  is  far  from  complete,  a  few 
words  may  be  devoted  to  this  subject,  as  it  will  often  be  of  use  to 
the  analyst  to  know  which  elements  should  be  especially  looked 
for  and  which  may  safely  be  neglected.1  The  various  minerals 
which  carry  the  several  elements  in  question  will  also  be  mentioned 
as  well  as  the  amounts  in  which  the  elements  usually  occur. 

Titanium  is  invariably  present,  as  shown  by  all  rock  analyses 
in  which  it  has  been  sought  for,  and  it  is  ninth  in  order  of  abun- 
dance among  the  elements  making  up  the  crust  of  the  earth.  Its 
amount  is  very  small  in  the  more  quartzose  and  feldspathic  rocks 
but  it  is  most  abundant  in  the  more  femic  rocks.  It  is  an  essential 
component  of  ilmenite  and  perofskite,  which  are  most  common  in 
femic  rocks,  and  of  titanite,  which  occurs  in  rocks  with  more  silica. 
The  oxide  rutile  is  rare  in  igneous  rocks,  but  is  common  in  meta- 
morphic  ones.  Titanium  also  enters  into  some  magnetite  and 
hematite,  and  also  very  commonly,  but  to  a  less  extent,  into 
pyroxenes,  amphiboles,  biotites,  garnets  and  other  ferromagnesian 
minerals.  Its  amount  in  rocks  may  vary  from  less  than,  and 
seldom  over,  one  per  cent,  to  five  or  more  per  cent  reckoned  as 
Ti02. 

Zirconium  is  present  in  many  rocks  in  small  amount,  and 
is  apt  to  occur  in  granites,  pegmatites,  rhyolites,  syenites,  and 
in  nephelite  syenites,  phonolites,  and  tinguaites.  It  is  most 

!See  also  F.  W.  Clarke,  Bull.  U.  S.  Geol.  Surv.,  78,  pp.  34^2,  1891; 
J.  H.  L.  Vogt,  Zeits.  Prakt.  Geol.,  1898,  pp.  225  ff.;  F.  W.  Clarke,  Bull. 
U.  S.  Geol.  Surv.,  168,  pp.  13-16,  1900;  Bull.  U.  S.  Geol.  Surv.,  616,  pp.  13, 
39,  1916.  J.  F.  Kemp,  Ore  Deposits  of  the  United  States,  New  York,  1900, 
p.  35;  Hillebrand,  Bull.  422,  p.  23;  H.  S.  Washington,  Trans.  Am.  Inst. 
Min.  Eng.,  30,  p.  809,  1908;  J.  P.  Iddings,  Igneous  Rocks,  1,  1909,  p.  27. 


18  INTRODUCTION 

abundant  in  rocks  which  are  high  in  soda,  such  as  the  last  three. 
It  is  rarely  met  with  in  rocks,  rich  in  lime,  magnesia,  and  iron. 
Zirconium  is  usually  found  as  the  silicate  zircon,  especially  in 
granites  and  syenites,  but  is  also  an  ingredient  of  such  rare  minerals 
as  eudialyte,  lavenite,  and  rosenbuschite.  Zirconium  is  present 
usually  in  amounts  less  than  0.20  per  cent  of  ZrO2,  but  may  rarely 
reach  2  per  cent  or  more. 

.  The  rare  earths,  oxides  of  cerium  and  yttrium,  and  of  their 
congeners,  are  found,  so  far  as  known  in  notable  amount  only  in 
rocks  that  are  high  in  soda.  They  form  part  of  allanite,  a  mineral 
that  is  rather  widespread  in  granites,  and  also  of  monazite,  mosan- 
drite,  xenotime,  and  other  minerals  of  even  greater  rarity.  They 
are  almost  always  accompanied  by  notable  amounts  of  zirconia. 
The  rare  earths,  reckoned  as  (Ce,  Y^Os,  seldom  are  present  in 
more  than  one-tenth  of  one  per  cent,  but  two  rocks  are  known  in 
which  they  are  present  to  the  extent  of  about  0.4  and  0.6  per  cent. 
Scandium  has  been  shown  to  be  widespread  in  traces. 

Chromium  is  almost  wholly  confined  to  the  femic  rocks,  espe- 
cially those  which  are  high  in  magnesia  and  low  in  silica,  and  which 
consequently  contain  abundant  olivine,  such  as  peridotite  and 
dunite.  It  occurs  as  chromite  and  picotite  (chrome-spinel), 
and  in  a  few  augites,  biotites  and  olivines.  It  may  occur  up  to 
one-half  of  one  per  cent  of  C^Oa- 

Vanadium,  according  to  the  investigations  of  Hillebrand, 
"  predominates  in  the  less  siliceous  igneous  rocks  and  is  absent, 
or  nearly  so,  in  those  high  in  silica."  It  is  an  ingredient  of  pyrox- 
enes, hornblendes,  and  biotites,  but  not  of  olivine,  and  is  also 
present  in  the  ilmenite  of  some  titaniferous  iron  ores.  Its  amount 
is  always  very  small,  seldom  over  0.05  per  cent  of  V2Oa. 

Manganese  is  uniformly  present  in  nearly  all  rocks,  but  its 
amount  is  small,  generally  in  tenths  of  a  per  cent  as  MnO,  being 
only  very  exceptionally  one-half  of  one  per  cent  or  more.  The 
high  figures  commonly  reported  are,  in  most  cases,  regarded  as 
due  to  analytical  error,  an  opinion  in  which  Dr.  Hillebrand  con- 
curs. It  occurs  in  the  ferromagnesian  minerals,  pyroxenes,  amphi- 
boles,  micas,  olivines  and  garnets,  and  therefore  is  usually  present 
in  greatest  amount  in  the  more  femic  rocks.  It  seems  to  favor 
rocks  high  in  iron  rather  than  those  high  in  magnesia. 

Nickel  and  cobalt,  like  chromium,  are  most  abundant  in  olivine 


THE  OCCURRENCE  OF  VARIOUS  ELEMENTS      19 

rocks,  but  the  former  has  been  noted  in  certain  basalts.  They 
occur  as  ingredients  of  olivine,  as  well  as  in  pyrite  and  pyrrhotite, 
and  in  hornblende  and  biotite  to  a  small  extent.  The  amount 
of  nickel  in  terrestrial  rocks  is  seldom  more  than  0.05  or  0.10  per 
cent,  while  that  of  cobalt  is  only  exceptionally  more  than  a  trace. 
In  some  analyses  nickel  has  been  undoubtedly  reported  too  high, 
probably  having  been  confounded  with  contaminating  platinum. 

Copper  is  often  to  be  found  in  igneous  rocks,  but  as  a  rule 
merely  in  traces.  It  is  probable  that  the  copper  reported  in  some 
analyses  of  igneous  rocks  is  really  platinum  derived  from  the  plat- 
inum basin  and  crucibles,  while  again  it  may  be  due  to  contamina- 
tion from  the  copper  water  baths,  etc.  It  is  most  frequent  in 
the  more  femic  rocks,  such  as  diabase,  gabbro,  and  basalt,  entering 
the  pyroxene  and  amphibole,  as  well  as  being  a  component  of 
chalcopyrite,  and  in  traces  in  some  pyrite.  It  also  (rarely) 
occurs  as  the  metal. 

Barium  and  strontium  are  very  commonly  present  in  igneous 
rocks,  the  latter  uniformly  in  less  amount  than  the  former.  There 
is  some  evidence  that  barium  is  apt  to  be  most  abundant  in  rocks 
which  are  high  in  potash.  Barium  occurs  in  the  feldspars,  espe- 
cially orthoclase,  as  the  celsian  molecule,  in  the  rare  hyalophane,  in 
some  zeolites,  as  well  as  in  a  few  biotites  and  muscovites.  We  can, 
at  present,  form  no  definite  conclusion  as  to  the  character  of  the 
rocks  most  likely  to  carry  strontium,  and  more  analytical  data  on 
this  point  would  be  of  interest.  The  amount  of  BaO  may  reach 
1  per  cent,  though  it  is  usually  much  less,  while  that  of  SrO  may 
run  up  to  0.30  per  cent,  but,  as  a  rule,  is  little  more  than  a  trace. 
Lithium  is  an  element  of  very  widespread  occurrence,  but  is 
seldom  met  with  in  rocks  in  more  than  spectroscopic  traces.  It 
may  be  expected  to  be  most  abundant  in  highly  alkalic  rocks,  and 
there  is  reason  for  the  belief  that  it  is  especially  prone  to  occur  in 
sodic  ones.  Apart  from  its  occurrence  as  an  essential  constituent 
of  such  minerals  as  lepidolite  and  spodumene,  which  occur  in 
highly  silicic  pegmatites  and  granites,  it  is  also  found  in  the  alkali 
feldspars,  muscovite,  beryl,  and  other  minerals. 

Phosphorus  is  almost  invariably  present  in  igneous  and  meta- 
morphic  rocks,  like  titanium,  and  like  this  element  it  is  most 
abundant  in  the  more  femic  ones,  especially  in  those  which  are 
high  in  lime  and  iron  rather  than  in  magnesia.  It  occurs  almost 


20  INTRODUCTION 

solely  in  apatite,  or  very  exceptionally  as  xenotime  or  monazite. 
While  the  quantity  of  P20s  usually  runs  from  0.10  to  1.50  per 
cent,  generally  under  1  per  cent,  it  may  occasionally  amount  to 
much  more. 

Sulphur,  as  sulphides,  is  far  more  abundant  in  the  femic 
rocks  of  all  kinds  than  in  the  salic  ones,  and  forms  an  essential 
ingredient  of  pyrite  and  pyrrhotite,  and  the  rare  mineral  lazurite. 
As  sulphur  trioxide  (SOs)  it  occurs  in  the  minerals  haiiynite,  nose- 
lite,  and  lazurite,  and  usually  in  the  more  femic  rocks,  though 
some  haiiynite  rocks  carrying  quartz  are  known.  These  three 
minerals  are  most  apt  to  occur  in  rocks  which  are  high  in  soda. 
Sulphur,  as  sulphides,  is  present  usually  in  tenths  of  a  per  cent, 
as  is  also  true  of  80s,  though  in  certain  cases  the  amount  may  be 
much  higher.  It  is  a  common  error  to  report  sulphur  as  80s 
instead  of  as  S. 

Chlorine  is  present  most  abundantly  in  rocks  which  are  high 
in  soda,  and  especially  when  so  low  in  silica  that  nephelite  is 
present,  though  it  is  also  found  sometimes  in  nephelite-free 
rocks,  and  in  a  few  cases  in  quartz-bearing  ones.  It  is  an  essential 
component  of  sodalite  and  noselite,  and  is  also  present  in  scapolite 
and  in  a  few  apatites.  It  also  occurs,  as  sodium  chloride,  in  liquid 
inclusions.  The  amount  of  Cl  is  usually  a  few  tenths  of  a  per 
cent,  but  in  sodalite  rocks  it  may  be  1  per  cent  or  more. 

In  specimens  of  rocks  collected  near  the  sea  coast  the  presence 
of  chlorine  may  be  due  to  contamination  by  sea  water.  This 
should  be  ascertained  and  determined  by  leaching  the  rock  powder. 

Fluorine  as  a  component  of  apatite,  biotite,  etc.,  seems  to  have 
no  special  preference  as  to  magma,  though,  on  the  whole,  it  is 
found  more  frequently  in  silicic  than  in  femic  rocks.  It  is,  how- 
ever, most  apt  to  be  met  with  as  fluorite  and  some  other  rare 
fluorine-bearing  minerals,  in  rocks  that  contain  nephelite,  as 
foyaites  and  tinguaites.  It  is  an  essential  constituent  of  fluorite 
and  most  apatite,  and  as  an  integral  part  of  the  last  mineral  is 
almost  universally  present.  It  also  occurs  in  small  amount  in 
biotites  and  other  micas,  in  some  hornblende  and  augite,  as  well 
as  in  tourmaline,  topaz,  chondrodite,  etc.  Its  usual  amount  is 
very  small,  generally  from  traces  to  0.10  per  cent,  only  rarely 
being  above  the  latter  figure. 

Of  the  other  rare  elements  it  may  be  of  interest  to  note  the 


STATEMENT  OF  ANALYSES  21 

following.  Glucinum,  as  a  component  of  beryl  and  some  other 
very  rare  minerals,  is  most  frequent  in  granites,  pegmatites  and 
quartzose  gneisses;  it  seems  to  be  most  at  home  in  sodic  rocks. 
There  is  reason  to  think  that  the  high  alumina  sometimes  reported 
for  these  rocks  may  be  in  part  glucina,  which  has  not  been  sep- 
arated from  the  alumina,  and  due  to  the  presence  of  unidentified 
beryl.  This  is  a  point  worthy  of  investigation,  but,  so  far  as  I  am 
aware,  glucinum  has  never  been  looked  for  or  determined  in  an 
igneous  rock.  Tin,  as  the  oxide  cassiterite,  is  confined  to  the 
highly  silicic  rocks,  granites  and  pegmatites,  and  its  presence  is 
due  generally  to  pneumatolytic  processes.  It  also  may  occur 
in  traces  in  ilmenite,  micas,  and  feldspars.  Thorium  would  seem 
to  be  most  abundant  in  highly  sodic  rocks,  and  the  same  is  also 
apparently  true  of  radium  and  the  radioactive  elements.1  Molyb- 
denum, tungsten,  and  uranium  are  almost  exclusively  confined  to 
the  very  siliceous  rocks.  Zinc  has  been  met  with  in  granite,  as 
well  as  in  basic  rocks,  but  no  generalization  in  regard  to  it  is  pos- 
sible as  yet.  Platinum  is  found  almost  exclusively  in  peridotites, 
but  is  occasionally  met  with  in  connection  with  gabbros.  Boron, 
usually  as  a  constituent  of  tourmaline,  is  most  apt  to  occur  in 
highly  siliceous  rocks. 

6.  STATEMENT  OF  ANALYSES 

The  results  of  the  analysis  might  be  stated  in  terms  either 
of  the  elements  present  or  of  the  basic  oxides  and  acid  radicals.2 
While  the  former  may  be  the  more  logical  on  purely  theoretical 
grounds,  yet  the  latter  greatly  facilitates  calculations  based 
on  the  analytical  data,  and  being  universally  in  use,  renders 
comparison  of  all  rock  analyses  with  each  other  very  simple. 
It  should  therefore  be  adopted  without  question. 

The  order  in  which  the  constituents  are  tabulated  varies 
somewhat  widely  though  now  much  less  so  than  formerly.  In 
some  cases  the  order  is  roughly  that  in  which  the  constituents  are 
determined  in  the  course  of  the  analysis.  Elsewhere  one  finds 
the  acid  radicals  placed  first,  followed  by  the  basic  oxides.  Or 
SiO2  is  followed  immediately  by  AbOa,  or  sometimes  first  by  Ti02, 


1  Cf.  A.  Holmes,  Geol.  Mag.,  (6),  2,  p.  63,  1915. 
2Cf.  Mellor,  p.  251;  Ostwald,  pp.  212-215. 


22  INTRODUCTION 

and  then  by  the  more  important  basic  oxides,  generally  including 
MnO,  with  the  less  abundant  constituents  following  these. 

There  is  general  unanimity  only  in  heading  the  list  with  SiO2. 
In  regard  to  all  the  other  substances  reported  there  has  been  very- 
considerable  diversity  in  the  details  of  sequence.  Thus  CaO 
sometimes  precedes  and  sometimes  follows  MgO,  and  the  same 
is  true  of  Na2O  and  K2O.  This  lack  of  uniformity  is  to  be  de- 
plored, as  it  is  not  only  extremely  apt  to  lead  to  error  in  copying 
analyses  the  order  of  statement  of  which  is  unfamiliar,  but  it 
also  renders  needlessly  difficult  the  comparison  of  two  or  more 
analyses  tabulated  according  to  different  systems. 

Some  years  ago  it  was  proposed  1  that  petrographers  and 
chemists  follow  a  definite  and  uniform  plan  in  the  statement 
of  the  analyses  of  rocks,  and  the  order  then  suggested  with  the 
reasons  for  its  adoption  are  briefly  given  here.  It  may  only 
be  added  that  no  cogent  reason  has  been  brought  forward  for  any 
important  modification,  and  that  it  has  been  adopted  in  its  essen- 
tials by  the  chemists  of  the  U.  S.  Geological  Survey.2 

The  general  foundation  for  the  order  proposed  is  that  analyses 
of  rocks  are  intended  primarily  for  the  benefit  of  petrographers 
and  petrologists,  so  that  an  arrangement  along  analytical  or  strictly 
chemical  lines  is  neither  advantageous  nor  appropriate.  To  them 
the  eight  oxides,  Si02,  A12O3,  Fe2O3,  FeO,  MgO,  CaO,  Na2O 
and  K20,  which  are  present  in  preponderating  amount,  in  the  vast 
majority  of  rocks  are,  and  must  always  remain,  of  prime  impor- 
tance. H2O  and  C02,  which  are  also  often  present  to  a  very  notable 
extent,  are  of  value  as  measures  of  the  freshness  of  the  rock. 
The  other  constituents,  while  of  varying  interest,  are  usually 
present  in  small  or  minute  quantities,  and  influence  the  character 
of  the  rock  only  to  a  limited  extent.  The  order  suggested,  with  a 
few  slight  modifications,  is : 

Si02,  A12O3,  Fe2O3,  FeO,  MgO,  CaO,  Na2O,  K2O,  H2O+ 
(>110°),  H20-(<110°),  C02,  Ti02,  Zr02,  P2O5,  B2O3,  SO3, 
Cl,  F,  S(FeS2),  (Ce,  Y)203,  Cr2O3,  V2O3,  MnO,  NiO,  CoO,  CuO, 
(ZnO),  BaO,  SrO,  Li2O. 

By  putting  the  eight  main  oxides  together  and  at  the  head, 

1 H.  S.  Washington,  Am.  J.  Sci.,  10,  p.  59,  1900. 

2  The  only  noteworthy  difference  is  that  in  the  practice  of  the  Survey 
the  positions  of  H2O+  and  H2O—  are  interchanged. 


STATEMENT  OF  ANALYSES  23 

the  general  character  of  the  rock  may  be  seen  at  a  glance.  Further- 
more, whether  an  analysis  is  complete  or  incomplete,  these  oxides 
are  always  in  the  same  relative  position,  and,  as  they  are  (or 
should  be)  determined  in  every  case,  the  eye  finds  them  without 
trouble,  thus  greatly  facilitating  comparison  and  study. 

As  regards  the  main  portion,  we  start  out  with  the  chief  acid 
radical  and  the  constituent  which  is  present  in  largest  amount, 
and  pass  through  successively  lower  orders  of  oxides  to  the  most 
positive  bases,  the  alkalies.  At  the  same  time  they  are  presented 
in  a  way  which  brings  the  oxides  together  in  their  natural  petro- 
graphic  and  mineralogic  relations.  Alumina,  which  often  appa- 
rently has  an  acidic  function  and  which  is  usually  the  most  abun- 
dant constituent  next  to  silica,  follows  immediately  after  this,  and 
is  succeeded  by  the  other  main  sesquioxide,  ferric  oxide.  Ferrous 
oxide  follows  ferric,  and  magnesia  is  next  to  it,  as  the  two  go  hand 
in  hand  in  the  ferromagnesian  minerals.  Lime  comes  next  in  an 
intermediate  position  between  these  and  the  alkalies,  as  is  proper, 
because  it  is  a  constituent  both  of  the  ferromagnesian  minerals 
and  of  the  feldspars.  Soda  precedes  potash,  as  it  is  associated 
with  lime  in  the  plagioclases. 

Water  follows  immediately  after  the  main  oxides,  since  it  is 
an  important  and  a  generally  determined  constituent.  Com- 
bined water  precedes  hygroscopic,  being  the  more  important 
and  almost  invariably  present  in  greater  amount  than  the  latter. 
Carbon  dioxide  comes  next,  as  it,  with  water,  is  a  measure  of  the 
freshness  of  the  rock,  and  this  character  can  therefore  be  told  at  a 
glance.  They  also  constitute  together  the  "  loss  on  ignition  " 
so  frequently  given,  and  may  then  be  connected  by  a  bracket  in 
comparative  tabulations. 

Of  the  minor  constituents  the  acid  radicals  come  first,  their 
sequence  following  the  main  principle  of  the  other  division. 
Titanium  and  zirconium  dioxides  are  placed  at  the  head,  as  they 
are  chemically  similar  to  silica,  and  often  replace  it.  Phosphorus 
pentoxide  comes  next  as  it  is  usually,  next  to  titanium  dioxide, 
the  most  important  and  abundant  of  the  minor  constituents. 
Boric  oxide  is  very  seldom  determined  but,  in  case  it  is  looked  for, 
its  proper  place  would  be  just  after  phosphorus  pentoxide.  Sul- 
phur trioxide  and  chlorine  are  together,  since  both  of  these  are 
constituents  of  the  sodalite  group  of  minerals.  Fluorine  follows 


24  INTRODUCTION 

immediately  after  chlorine,  both  being  halogens.  Sulphur  com- 
pletes the  list  of  the  minor  acid  radicals,  being  less  acidic  than 
the  others;  it  is  also  frequently  present  as  a  constituent  of  appa- 
rently secondary  origin  and  is  thus  analogous  to  water  and  carbon 
dioxide  among  the  main  constituents. 

The  subordinate  metallic  oxides  follow  in  the  order  R2Oa, 
RO,  and  R2O.  Among  the  sesquioxides  those  of  the  rare  earths 
^though  they  are  seldom  determined),  come  first,  as  they  appear 
to  be  isomorphous  with  alumina  mineralogically.  Chromium 
sesquioxide  precedes  vanadium  as  it  is  the  more  important. 
These  two  might  be  placed  among  the  minor  acid  radicals,  but  the 
position  chosen  seems  the  best.  Manganous  oxide  precedes  the 
oxides  of  nickel  and  cobalt,  as  it  is  very  frequently  determined, 
and  is  present  in  greater  amount.  .The  monoxides  of  the  other 
heavy  metals  when  present  come  next,  those  just  mentioned  pre- 
ceding on  account  of  their  greater  importance  and  their  chemical 
affinity  with  ferrous  oxide.  Of  the  oxides  of  the  minor  alkali- 
earth  metals,  which  are  next  in  order,  baryta  precedes  strontia  as 
the  more  abundant  and  important.  Lithia  closes  the  list  as  the 
only  minor  representative  of  the  alkali  metals. 

In  publishing  the  analysis  it  may  be  recommended  that  the 
molecular  numbers  of  each  of  the  constituents,  (obtained  by  divid- 
ing the  percentage  amount  by  the  molecular  weight) ,  be  given 
along  with  the  regular  statement.  The  user  of  the  analysis 
will  thus  be  saved  the  trouble  of  calculating  them  for  himself, 
and  the  chemical  character  of  the  rock  will  be  more  fully  and 
immediately  comprehended. 

In  the  statement  of  analyses  the  term  "  trace  "  is  in  frequent 
use,  to  indicate  that  a  constituent  is  present,  or  supposed  to  be 
present,  in  a  small  but  undetermined  amount.  The  use  of  the 
term  has  been  loose,  and  in  some  rocks  quite  erroneous,  as  more 
complete  analyses  have  shown  that  such  "  traces  "  may  amount 
in  reality  to  one-half  of  one,  or  possibly  to  several  per  cent.  It 
would  be  better  to  have  the  meaning  of  the  term  more  strictly 
defined,  and  it  has  been  suggested  l  that  it  "  should  indicate 
strictly  and  uniformly  that  the  constituent  (to  which  it  is  applied) 
has  been  looked  for  and  found,  but  in  unweighable  amount  (0.1 
milligram  or  less)."  If  there  is  no  knowledge  as  to  the  presence 
1  H.  S.  Washington,  Prof.  Paper  99,  p.  16. 


STATEMENT  OF  ANALYSES  25 

of  a  substance  a  dash  may  be  used.  Hillebrand  suggests  that, 
"  In  the  tabulation  of  analyses  a  special  note  should  be  made  in 
case  of  intentional  or  accidental  neglect  to  look  for  substances 
which  it  is  known  are  likely  to  be  present."  For  this  purpose  the 
letters  "  n.  d."  (not  determined)  may  be  reserved.  The  absence 
of  any  constituent,  if  looked  for,  should  always  be  stated  in  the 
tabulation  and  not  in  the  accompanying  text,  as  is  sometimes 
done,  since  the  fact  is  thus  apt  to  be  overlooked. 

The  analytical  calculations  should  be  carried  to  four  decimals, 
which  means  that  in  the  statement  of  analyses  the  figures  are  to 
be  given  to  hundredths  of  a  per  cent.  While  the  last  decimal 
may  not  be  of  much  significance,  it  represents  the  limit  of  weighing 
(0.0001  gram)  in  the  quantities  taken  for  the  determination  of  the 
constituents  of  rocks,  and  gives  some  assurance  of  the  value  of 
the  preceding  decimal.  It  is  also  the  almost  universal  practice 
among  chemists  and  analysts.1  Statement  in  only  tenths  of  a  per 
cent  is  defective  in  that  it  implies  correctness  only  in  the  unit 
column,  and  consequently  an  insufficient  degree  of  accuracy. 
It  is  also  of  an  inadequate  degree  of  accuracy  for  the  minor 
constituents. 

On  the  other  hand,  a  statement  in  thousandths  of  a  per  cent 
implies  a  higher  degree  of  accuracy  than  is  possible  with  the  limits 
of  error  obtaining  in  all  but  the  most  painstaking  analytical  work, 
and  which  is  quite  uncalled  for  in  view  of  the  somewhat  variable 
composition  of  all  rock  masses  from  place  to  place,  however  great 
may  be  the  apparent  uniformity.  It  may  be  remarked  that,  in 
the  course  of  compiling  and  examining  thousands  of  rock  analyses, 
I  have  found  it  to  be  true,  almost  without  exception,  that  the  few 
analyses  given  to  thousandths  of  a  per  cent  are  remarkable  chiefly 
for  their  poor  quality,  differing  from  the  probable  truth  in  some  or 
all  constituents  by  as  much  as  one  or  more  per  cent.  Statement 
in  such  ultra-refined  terms  may  usually  be  regarded  as  evidence 
that  the  analyst  has  no  just  appreciation  of  the  probable  limits  of 
error,  or  of  the  bases  of  accuracy  in  analytical  work. 

A  final  word  must  be  said  in'  regard  to  the  recalculation  of 

1  This  topic  is  well  discussed  by  Mellor  (p.  16),  who  says:  "  As  a  matter  of 
fact,  two  decimals  are  generally  used  in  technical  analyses  because  we  have 
grown  accustomed  to  the  plan,  not  because  it  represents  the  accuracy  of  the 
work."  Cf.  Washington,  Prof.  Paper,  99,  p.  22. 


26  INTRODUCTION 

the  analysis  to  an  even  100  per  cent.1  This  is  tantamount  to  the 
distribution  of  any  errors  over  all  the  constituents,  which  is  not 
justifiable,  as  has  been  said  elsewhere.  Furthermore,  as  Fresenius 
says,  "  such  '  doctoring '  of  the  analysis  deprives  other  chemists 
of  the  power  of  judging  of  its  accuracy."  Whatever  the  results 
may  be,  and  whether  the  summation  be  high  or  low,  the  figures 
for  the  various  constituents  should  be  given  with  their  summation, 
as  they  are  obtained  from  the  analysis,  if  the  whole  is  deemed  to 
be  worthy  of  publication  at  all.  Any  other  procedure  would  give 
rise  to  reasonable  suspicion  as  to  the  accuracy  of  the  analysis, 
which  can  only  be  judged  of  by  others  if  the  actual  figures  are  given. 

1  Cf .  Mellor,  p.  246. 


PART  II 

APPARATUS  AND  REAGENTS 

ALTHOUGH  any  well-equipped  laboratory  should  have  nearly 
every  piece  of  apparatus  and  all  the  reagents  which  are  necessary 
for  the  quantitative  analysis  of  rocks,  it  may  be  convenient,  espe- 
cially for  the  independent  worker,  to  give  a  list  of  those  that  should 
be  available,  and  which  will  be  needed,  during  the  progress  of  a 
properly  carried  out  analysis.  Brief  remarks  will  be  made  on 
certain  points  which  it  is  useful  for  the  inexperienced  to  know. 
The  number  of  pieces  of  apparatus  suggested  are  those  which  it 
is  deemed  advisable  to  have  at  hand,  so  that  a  series  of  analyses 
may  proceed  without  interruption  for  lack  of  adequate  facilities; 
and  the  amounts  of  reagents  suggested  are  such  as  it  will  be  well 
to  provide  when  stocking  up  an  individual  laboratory. 

Suggestions  as  to  general  laboratory  equipment  are  not  made, 
as  this  is  usually  provided  for  the  student  and  can  seldom  be  rad- 
ically changed.  It  may  be  well,  however,  to  emphasize  the 
importance  of  a  hood  with  a  good  draught,  provision  of  a  steam- 
bath,  an  efficient  arrangement  for  suction,  and,  above  all,  proper 
arrangements  for  keeping  the  laboratory  clean  and  free  from  dust 
and  fumes. 

1.  APPARATUS  l 

BALANCE   AND   WEIGHTS2 

Balance. — A  good,  reliable,  and  accurate  balance  and  weights 
are  essential  to  good  analytical  work.  It  cannot  be  impressed 

1  In  connection  with  many  of  the  pieces  of  apparatus  references  are  made 
to  the  figures  in  the  catalogues  of  Eimer  &  Amend,  New  York,  1913  edition 
(E.  &  A.),  and  of  Arthur  H.  Thomas  Company,  Philadelphia,  edition  of  1914 
(A.  T.  Co.),  so  that  the  correct  form  can  be  identified. 

2Cf.  Fresenius,  1,  p.  12;  Gooch,  p.  11;  Mellor,  p.  3;  Morse,  p.  1;  Tread- 
well,  2,  p.  6;  P.  J.  Krayer,  The  Use  and  Care  of  a  Balance.  Easton,  Pa.,  1913. 

27 


28  APPARATUS  AND  REAGENTS 

too  strongly  on  the  beginner  that  the  value  of  his  analytical  work 
rests  fundamentally  on  the  quality  of  his  balance  and  weights  and 
on  the  care  with  which  they  are  treated.  The  other  apparatus 
may  be  adequate,  the  reagents  of  the  utmost  purity,  the  methods 
of  the  best,  and  the  manipulation  careful,  delicate,  and  conscien- 
tious; but  if  the  balance  and  weights  are  not  accurate,  and  are  not 
carefully  taken  care  of,  the  labor  and  time  expended  on  the  analysis 
will  largely  or  wholly  go  for  naught.  The  balance  and  weights 
should,  therefore,  be  regarded  with  a  feeling  akin  to  reverence,  and 
the  balance-case  be  looked  upon,  so  to  speak,  as  a  sanctum  sanc- 
torum. 

There  are  several  good  balances,  of  moderate  cost,  on  the  mar- 
ket that  will  answer  all  the  requirements  for  any  but  the  most 
exacting  work.  Only  a  high-grade,  analytical  (not  a  so-called 
"  student's  ")  balance,  of  a  reliable  maker,  should  be  selected. 

The  capacity  should  be  200  grams,  with  a  sensibility  of  one- 
tenth  of  a  milligram  at  full  load.  The  bearings  should  be  of  agate, 
the  beam  of  magnalium  or  some  such  light  and  not  easily  cor- 
rodible  metal,  and  graduated  for  a  rider. 

As  summarized  by  Mellor,  the  conditions  which  must  be  satis- 
fied by  a  good  balance  are:  1,  it  must  be  consistent,  that  is,  give 
the  same  result  in  successive  weighings  of  the  same  body;  2,  it 
must  be  accurate;  3,  it  must  be  stable,  so  that  the  beam  after  being 
displaced  will  return  to  its  horizontal  position ;  4,  it  must  be  sensi- 
tive; 5,  in  order  to  avoid  loss  of  time  in  weighing,  the  beam  must 
oscillate  quickly,  and  must  therefore  be  short. 

It  will  be  convenient  to  adjust  the  sliding  or  screw  weight  on 
the  pointer  or  beam  so  that  one  (small)  division  of  the  scale  will 
correspond  to  0.1  milligram.  It  is  important  that  the  zero  point 
be  determined  from  time  to  time.  For  most  work  it  is  not  nec- 
essary to  determine  it  before  each  weighing,  as  is  recommended  by 
some,  but  it  should  be  borne  in  mind  that  some  balances  are 
liable  to  change  of  zero  point  with  change  of  temperature. 

The  balance  must  be,  of  course,  in  a  case,  with  appropriate 
releases  for  the  knives  and  pans.  The  case  is  always  to  be  kept 
closed  when  not  in  use.  A  2-inch  funnel,  filled  with  granular 
calcium  chloride,  and  supported  in  a  small  Erlenmeyer  flask,  is 
to  be  kept  in  the  back  part  of  the  case.  Sulphuric  acid  should 
never  be  used  as  a  desiccating  agent  in  a  balance.  The  case  and 


APPARATUS  29 

pans  are  to  be  kept  free  from  dust  by  light  brushing  with  a 
camePs-hair  brush  from  time  to  time. 

In  reading  the  pointer  it  is  convenient  to  use  a  "  balance 
reading  glass  "  mounted  in  front  of  the  ivory  scale.  For  setting 
the  beam  to  swinging  a  very  useful  adjunct  is  a  rubber  hand-bulb, 
connected  with  a  tube  passing  through  the  floor  of  the  case,  and 
permitting  a  puff  of  air  to  be  blown  against  the  bottom  of  one  of 
the  pans.1  The  balance  should  also  be  provided  with  a  small, 
light  metal  stand  for  holding  tubes,  as  well  as  a  shelf  to  straddle 
the  pan  for  specific  gravity  determinations. 

The  location  of  the  balance  is  a  matter  of  importance.  It  is 
discussed  by  Morse,  and  we  can  hardly  do  better  than  summarize 
what  he  says.  The  essential  points  are : 

1.  The  balance  case  should  be  in  a  room  separate  from  the 
laboratory,  where  it  is  free  from  corroding  gases. 

2.  It  should  be  so  located  that  the  two  arms  will  maintain, 
as  nearly  as  possible,  the  same  temperature  and  consequently 
the  same  length.     Hence  it  should  not  be  placed  near  a  window, 
especially  one  with  southern  exposure,  or  a  source  of  heat,  such  as  a 
radiator,  stove,  or  hot  air  vent.     It  must  not  be  placed  where  it 
will  be  in  direct  (or  even  reflected)  sunlight  at  any  time  of  the  day 
or  year. 

3.  The  source  of  light,  especially  if  artificial,  should  be  above 
and  back  of  the  head  of  the  observer,  and  so  located  with  respect 
to  the  beam  that  the  heat  from  it  will  affect  both  arms  equally. 
Diffuse  daylight  from  a  window  at  one  side,  if  somewhat  in  the 
rear  of  the  observer  and  not  very  near,  will  answer. 

4.  The  foundation  upon  which  the  balance  sets  should  be  as 
firm  as  possible,  and  any  jarring  or  vibration  from  machinery 
is  to  be  avoided.     A  heavy  table  with  stout  legs  (which  may  be 
made  of  iron  piping),  and  with  a  stone  slab  for  a  top  answers  well. 
The  top  must  be  strictly  horizontal.     There  should  be  sufficient 
space  on  it  for  the  note-book  and  desiccator. 

For  cautions  as  to  the  use  of  the  balance,  see  the  section  on 
weighing,  pp.  79,  129. 

Weights. — The  weights  should  be  of  the  "  first  quality  "  of 

1  So  far  as  I  know  this  is  provided  only  with  the  Riiprecht  balances.  It 
should  be  better  known  and  more  widely  used,  as  it  is,  by  far,  the  best  device 
for  its  purpose. 


30  APPARATUS  AND  REAGENTS 

the  catalogues.  The  set  should  run  from  at  least  50,  or  better 
100,  grams  down  to  1  centigram.  The  weights  below  the  10  milli- 
gram weight  are  not  needed,  as  the  rider  is  always  used  instead  of 
them.  The  gram  weights  are  best  made  of  brass,  preferably  gold 
or  platinum  plated,  though  Krayer  advocates  the  use  of  first- 
quality  lacquered  brass  weights.  The  smaller  weights  are  of 
platinum.  The  weights  may  be  kept  in  their  box  (covered)  in 
front  of  the  case,  though  some  prefer  to  keep  them,  arranged  in 
order,  in  the  balance  case  in  front  of  the  right-hand  pan. 

The  weights  should  never  be  touched  with  the  fingers,  but  are 
always  to  be  handled  with  the  ivory-tipped  forceps  that  should 
accompany  them.  The  handling  with  these  should  be  most  gentle, 
and  there  must  be  no  rubbing  or  scraping  of  the  neck  of  the  larger 
weights  in  putting  them  back  in  their  places.  They  must  be  kept 
free  from  the  action  of  any  dust  or  corroding  vapors,  and  must 
never  come  in  contact  with  any  solid  or  liquid  chemicals.  If  kept 
in  the  box,  this  is  always  to  be  closed  after  use.  The  weights  are 
to  be  occasionally  lightly  wiped  (not  rubbed)  with  a  silk  hand- 
kerchief. 

For  accurate  work  the  weights  should  be  tested.  The  descrip- 
tion of  this  process  is  rather  too  long,  and  the  student  is  referred 
to  Gooch  (p.  21),  Mellor  (p.  16),  Morse  (p.  26),  or  Treadwell 
(p.  15). 

It  is  assumed  that  the  laboratory  has  a  cheaper  and  less  accu- 
rate balance,  capable  of  weighing  up  to  1  kilogram  or  so,  and  set 
of  weights,  to  weigh  out  reagents  roughly  and  in  large  amounts. 

PLATINUM  l 

A  number  of  utensils  of  platinum  or  of  a  substitute  for  it  are 
indispensable  for  analytical  work.  The  present  price  of  platinum 
is  so  high  that  for  many  chemists,  unless  they  are  the  fortunate 
possessors  of  an  old  stock,  a  substitute  must  be  used.  For- 
tunately, this  is  to  be  found  in  the  recently  introduced  alloy 
(80  gold,  20  palladium),  called  "  palau."  According  to  the  inves- 
tigations of  the  Bureau  of  Standards,  this  compares  very  favor- 
ably with  platinum,  and  practically  all  the  usual  analytical  opera- 
tions that  are  carried  out  at  temperatures  not  much  above 

1  For  a  report  by  Hillebrand,  Walker,  and  Allen  on  the  quality  of  recent 
platinum  see  Jour.  Ind.  Eng.  Chem.,  3,  p.  686,  1911. 


APPARATUS  31 

1250°  C.  (approximately  that  of  an  ordinary  blast)  can  be  done 
equally  well  in  palau.  The  only  exception  would  be,  apparently, 
the  fusion  with  pyrosulphate,  which  attacks  palau  somewhat  more 
than  it  does  platinum.  Palau,  however,  suffers  less  loss  of  weight 
by  vaporization  than  platinum.1  In  the  following  list,  therefore, 
and  throughout  this  book,  when  platinum  is  mentioned  it  will  be 
understood  that  palau  may  be  substituted  for  it,  with  the  possible 
exception  of  the  crucible  for  the  pyrosulphate  fusion  (p.  159). 

List  of  Apparatus. — One  basin,  lipped,  of  about  300  c.c.  capa- 
city, 10  to  11  cm.  across  the  top,  and  about  5  cm.  deep,  weight 
about  100  grams.  This  should  not  be  hemispherical,  but  with  a 
somewhat  flatly  rounded  bottom  and  vertical  upper  sides  (E.  & 
A.,  5340,  A.  T.  Co.,  44116).  If  the  edge  is  stiffened  with  wire 
this  should  not  be  continued  around  the  lip  or,  if  so,  the  turned 
edge  should  be  soldered  with  platinum  (or  palau)  around  this,  to 
prevent  entrance  of  liquid  and  possible  contamination  or  loss. 
Instead  of  a  platinum  basin  one  of  chemically  pure  gold  (free  from 
copper)  will  serve,  and  good  porcelain  may  be  used  in  work  that  is 
not  very  accurate,  though  it  is  difficult  to  remove  all  silica  from 
this.  Neither  fused  silica  nor  glass  should  be  used  for  the  main 
evaporation  to  determine  silica. 

Four  crucibles;  two  of  about  35  c.c.  capacity,  and  two  of  about 
25  c.c.  Each  should  have  its  own  cover,  with  which  it  is  always 
weighed.  For  the  main  fusion  with  sodium  carbonate  the  cru- 
cible, if  of  platinum,  should  be  of  metal  alloyed  with  rhodium  or 
iridium  to  give  stiffness  (p.  132).  The  slight  loss  of  weight  due 
to  volatilization  is  of  no  moment  here,  as  the  weight  of  the  crucible 
after  the  fusion  is  not  taken.  Palau  is  sufficiently  stiff  to  do  well 
for  this  fusion.  One  of  the  25  c.c.  crucibles,  for  the  pyrosulphate 
fusion,  would  best  be  of  pure  platinum. 

One  Gooch  crucible,  3J-4  cm.  high,  3-3J  cm.  wide  at  the  top, 
and  2|-2|  at  the  bottom,  of  about  25  c.c.  capacity,  it  should  have 
a  cover.  A  close-fitting  cap  for  the  bottom  of  this  is  a  conveni- 
ence. A  porcelain  Gooch  may  be  used  instead  (p.  99). 

One  small,  shallow-lipped  basin,  of  about  50  c.c.  capacity,  and 
weighing  about  20  grams. 

One  spatula,  in  one  piece,  about  10  cm.  long  and  weighing  about 

1  For  a  study  of  the  quality  of  platinum  see  Burgess,  Sclent.  Papers,  Bur. 
Stand.,  Nos.  254  (1915)  and  280  (1916). 


32  APPARATUS  AND  REAGENTS 

10  grams.  It  should  be  rather  thick,  with  round,  broad  end,  and 
no  handle. 

Two  triangles,  one  of  5  and  one  of  6  cm.  along  the  side.  The 
style  with  twisted  ends  is  good,  if  the  ends  are  smooth  it  is  well  to 
make  a  series  of  small  notches  along  one  end  to  support  a  crucible 
cover  (p.  104). 

As  some  platinum  crucibles  may  adhere  to  platinum  triangles, 
triangles  of  clay  or  fused  silica  have  advantages  other  than  those 
of  low  cost,  and  are  indeed  recommended.  Those  made  of  silica 
tubing  strung  on  nickel  or  nichrome  wire  answer  well  in  place  of 
platinum.  If  one  has  both  platinum  and  palau  crucibles  and 
triangles  the  different  metals  must  never  be  in  contact  when  hot. 

One  perforated,  seamless  cone,  about  2.5  cm.  in  diameter. 

One  pair  of  crucible  tongs,  platinum  tipped,  of  brass  or  Ger- 
man silver,  and  of  the  usual  shape.  One  of  Blair's  form,  with 
platinum  or  palau  shoes,  will  also  be  useful. 

A  pair  of  blowpipe  forceps,  with  platinum  or  palau  tips,  is 
necessary  if  blowpiping  is  to  be  done,  and  is  very  useful  in  sepa- 
rating mineral  grains  for  analysis. 

A  few  small  pieces  of  platinum  foil,  cut  into  strips  and  bent  at 
right  angles  or  in  zigzags,  are  used  to  prevent  boiling  liquids  from 
"  bumping"  (p.  165). 

Care  of  Platinum. — All  or  nearly  all  platinum  and  palau  cruci- 
bles lose  weight l  (palau  less  than  platinum) ,  when  heated  above 
about  900°,  especially  on  blasting  (about  1200°).  With  platinum 
the  loss  is  apparently  due  in  part  to  the  iridium  content.  The 
rate  of  loss  for  each  crucible  used  should  be  ascertained;  or,  if 
serious,  the  empty  and  cleaned  (not  scrubbed  with  sand)  crucible 
should  be  weighed  after  an  ignition. 

There  are  several  precautions  to  be  observed  in  the  use  and 
care  of  platinum  2  (and  palau)  utensils. 

Platinum  utensils  must  be  kept  bright  and  clean;  this  applies 
especially  to  crucibles.  It  is  best  accomplished  by  rubbing  the 
crucible  with  sea  sand  moistened  with  water.  The  grains  must 
be  rounded  and  the  sand  free  from  grit.  I  have  found  sand  from 

1  Cf.  Hillebrand,  Bull.  422,  p.  94,  footnote;   Burgess  and  Sales,  Jour.  Ind. 
Eng.  Chem.,  6,  p.  452,  1914;  7,  p.  561,  1916;  Sci.  Pap.  Bur.  Stand.,  No.  254, 
1915;  Burgess  and  Walters,  Sci.  Pap.  Bur.  Stand.,  No.  280,  1916. 

2  Cf.  Mellor,  p.  115. 


APPARATUS  33 

the  New  Jersey  coast  excellent.  The  whole  lot  of  sand  should  be 
well  washed,  digested  with  warm,  dilute  hydrochloric  acid  for 
some  time,  to  remove  fragments  of  shells,  and  then  washed  again 
and  dried.  The  crucible  should  not  be  deformed  in  the  rubbing. 
A  crucible  should  be  sand-rubbed  quite  frequently,  especially 
after  a  long  ignition,  as  it  prevents  and  removes  surface  crystalli- 
zation and  coating,  and  greatly  prolongs  the  life  of  the  crucible. 

If  the  sand  rubbing  does  not  remove  firmly  adherent  particles 
of  ignited  precipitate  or  stains,  these  may  be  removed  by  fusing  a 
little  potassium  pyrosulphate  in  the  crucible.  If  the  stain  is  on 
the  exterior  the  salt  is  fused  in  a  larger  crucible  or  in  a  small  basin 
in  which  the  crucible  that  is  to  be  treated  is  immersed.  Because 
the  pyrosulphate  attacks  the  metal  this  cleansing  should  be  done 
as  seldom  as  possible  and  the  fusion  is  not  to  be  continued  longer 
than  necessary. 

Hot  platinum  must  never  be  touched  with  any  other  metal. 
Therefore  it  should  only  be  handled  with  the  platinum-tipped 
tongs,  and  should  not  rest  on  red-hot  wire  gauze,  whether  of 
iron  or  nichrome.  In  heating  the  platinum  basin  over  a  Bunsen 
burner  it  is  well  to  have  the  center  of  the  gauze  asbestos  covered. 
No  metals  or  salts  of  heavy  or  easily  reducible  metals  should  be 
heated  or  melted  in  it. 

Sulphides,  arsenides,  and  phosphides  should  not  be  heated  in 
platinum,  and  in  the  ignition  of  phosphates  care  should  be  taken 
that  no  reduction  takes  place. 

Fused  alkali  carbonates  attack  platinum  very  slightly  (p.  146), 
but  fused  caustic  alkalies,  as  well  as  fused  alkali  nitrates  and 
baryta,  attack  it  seriously.  Platinum  is  also  very  appreciably 
attacked  by  fused  potassium  pyrosulphate,  so  that  the  crucible 
shows  a  very  decided  loss  of  weight  after  each  fusion  with  this  salt. 

As  platinum  is  readily  attacked  by  free  chlorine,  platinum  must 
never  be  exposed  to  this.  Consequently,  it  should  never  come  in 
contact  with  a  mixture  of  hydrochloric  acid  and  a  strong  oxidizing 
agent  which  would  give  rise  to  free  chlorine.  If  manganates  are 
present,  they  should  be  reduced,  as  by  alcohol,  before  the  addition 
of  hydrochloric  acid. 

Platinum  should  not  be  exposed  to  burning  carbon,  as  coke  or 
coal,  though  the  flameless  burning  off  of  the  carbonized  filter  paper 
does  no  damage.  It  should  not  be  heated  in  a  smoky  or  luminous 


34  APPARATUS  AND  REAGENTS 

flame,  nor  in  the  inner  cone  of  the  Bunsen  burner.     The  air  supply 
of  this  must  be  so  adjusted  that  the  flame  is  non-luminous. 

Platinum  should  not  be  exposed  to  violent  changes  of  tem- 
perature, such  as  exposure  when  red-hot  to  a  blast  of  cold  air  or 
immersion  in  water,  and  the  crucible  must  not  be  bent  or  squeezed 
as  is  frequently  recommended,  to  loosen  a  fused  cake.  A  little 
care  and  patience  will  obviate  the  need  of  such  measures,  and  the 
loss  of  a  little  time  is  preferable  to  the  misfortune  of  a  dented  and 
distorted,  and  possibly  cracked  and  ruined,  crucible. 

GLASS 

At  the  present  time  the  formerly  much-used  Jena  and  Kavalier 
glasses  are  almost  unobtainable,  and  will  probably  continue  so 
for  years  to  come.  This,  however,  is  of  no  consequence,  as 
there  are  now  produced  in  the  United  States  1  several  makes  of 
chemical  glassware  that  are  equal  to,  and  in  some  respects  excel, 
the  best  of  the  German  glasses,  such  as  Pyrex,  Nonsol,  Insol, 
Fry  and  others.  The  Pyrex  is  especially  valuable  because  of  its 
very  low  coefficient  of  expansion,  which  permits  thicker  walls  and 
consequently  less  liability  to  breakage,  either  mechanically  or  by 
sudden  changes  of  temperature.  Official  tests  2  have  shown  that 
such  glasses  are  entirely  satisfactory  in  all  essential  respects,  such 
as  resistance  to  solutions  of  acids,  alkalies,  and  various  salts, 
and  to  hot  and  cold  water,  and  sudden  changes  of  temperature, 
proper  annealing,  correct  shape,  and  uniformity.  All  beakers  and 
flasks  used  in  the  analysis  should  be  of  such  resistant  glass. 

Beakers  should  be  on  hand  in  liberal  quantity.  They  should 
all  be  lipped  and  of  the  usual  form;  the  low  (Griffin's)  and  the  tall 
forms  are  seldom  if  ever  needed.  It  is  convenient,  but  not 
necessary,  to  have  a  small  space  ground  or  etched  on  the  side 
for  writing  on.  The  following  number  of  each  size  would  best 
be  kept  at  hand:  Three  1000  c.c.,  four  800  c.c.,  five  600  c.c.,  six 
600  c.c.,  six  250  c.c.,  five  200  c.c.,  ten  150  c.c.,  ten  100  c.c.,  five 
50  c.c. 

Burettes. — It  is  best  to  have  two  types  of  these.  There  should 
be  two  Ripper's3  weight-burettes  (E.  &  A.,  1354),  one  of  50  and 
1 1  understand  that  similar  glass  is  now  made  in  England. 

2  See  Walker  and  Smither,  Jour.  Ind.  Eng.  Chem.,  9,  p.  1050,  1917. 

3  M.  Ripper,  Chem.  Zeit.,  p.  793,  1892. 


APPARATUS  35 

one  of  100  c.c.,  for  the  iron  determinations.  These  should  have 
glass  caps  to  cover  the  tips,  and  a  wire  loop  to  support  them  on 
the  hook  above  the  balance  pan. 

Some  of  the  advantages  of  a  weight-burette  over  one  of  the 
usual  form  are : l  Correction  for  temperature  changes  is  not  needed, 
adherence  of  liquid  to  the  walls  is  of  no  consequence,  the  solution 
can  be  weighed  readily  to  0.01  gram,  whereas  measurement  to  0.01 
c.c.  is  very  uncertain. 

If  the  burettes  become  stained  brown  by  manganese,  the 
stain  can  be  readily  removed  with  a  little  solution  of  sulphur 
dioxide.  The  burette  is  to  be  very  thoroughly  washed  out  after 
its  use. 

There  should  be  also  two  burettes  of  the  ordinary,  Mohr's 
type  (E.  &  A.,  1314,  A.  T.  Co.,  22,556),  with  glass  stopcock, 
preferably  set  at  an  angle  of  90°.  These  are  of  50  c.c.,  graduated 
to  tenths  of  a  cubic  centimeter,  and  should  preferably  be  "  certi- 
fied." Such  devices  as  Erdmann's  float,  meniscus  reader,  and 
enamelled  strip  on  the  back  are  not  needed. 

Carbon  Filter  Tube.— (E.  &  A.,  3228,  A.  T.  Co.,  28,036).  One 
should  be  selected  of  a  size  to  fit  the  Gooch  crucible  that  is  in  use, 
with  allowance  for  the  rubber  connection.  The  tube  is  to  be  pro- 
vided with  a  singly  perforated  rubber  stopper  to  fit  the  filtering 
flask.  A  rubber  filtering  gasket  (A.  T.  Co.,  27,749)  or  crucible 
holder  may  be  used  instead  with  an  ordinary  funnel. 

Desiccators. — One  desiccator,  either  of  the  Fresenius  (E.  &  A., 
2546,  A.  T.  Co.,  25,870)  or  the  Scheibler  (E.  &  A.,  2528,  A.  T.  Co., 
25,830)  type,  with  a  pipe  stem  triangle,  serves  for  general  use. 
Granulated  soda-lime  or  calcium  chloride  should  be  used  as  the 
drying  agent,  as  sulphuric  acid  may  give  off  sulphur  dioxide  by 
the  action  of  dust.  It  is  convenient  to  have  also  a  larger,  Scheibler, 
desiccator,  containing  a  porcelain  plate  with  several  holes,  in  which 
crucibles,  etc.,  can  be  put  aside.  A  desiccator  with  a  stopcock 
for  drying  in  vacuo  is  sometimes  useful,  but  is  not  necessary  for 
general  work. 

Drying  Cylinders. — A  good  type  is  that  with  perforated  glass 
stopper  and  side  tube  near  the  top  (E.  &  A.,  7128,  A.  T.  Co., 
23,200).  Two  or  three,  of  medium  sizes,  will  do. 

!R.  S.  McBride,  Bull.  Bur.  Stand.,  8,  p.  617,  1912;  see  also  E.  W.  Wash- 
burn,  Jour.  Am.  Chem.  Soc.,  30,  p.  40,  1908. 


36  APPARATUS  AND  REAGENTS 

Drying  Tubes. — These  are  best  of  the  U  shape,  and  preferably 
of  the  Schwartz  type.  Three  or  four,  of  medium  sizes,  will  serve. 

Flasks,  both  of  the  ordinary,  flat-bottomed  and  Erlenmeyer 
shapes,  may  be  of  the  following  sizes:  Ordinary,  flat-bottom; 
two  each  of  400,  200,  100,  and  50  c.c.;  Erlenmeyer,  three  each  of 
500,  300,  and  200  c.c. 

Flask,  Filtering. — This  should  be  of  the  Erlenmeyer  shape, 
thick-walled,  and  with  a  side  tube  (E.  &  A.,  3090,  A.  T.  Co., 
28,248).  The  capacity  should  be  500  c.c. 

Funnels. — There  should  be  two  of  75  mm.,  three  of  65  mm., 
and  two  of  50  mm.  in  diameter.  Care  should  be  taken  to  select 
these  with  an  apical  angle  of  exactly  60°  and  perfectly  straight 
sides. 

The  ground-off  point  is  to  be  cut  off,  and  a  tube  about  22  cm. 
long,  and  internal  bore  of  3  mm.,  bent  into  a  circle  of  2  to  2J  cm. 
near  the  upper  end,  is  fused  to  the  stem  of  each.  It  is  important 
that  the  junction  be  smooth  and  without  enlargement.  This  tube 
greatly  promotes  rapidity  of  filtration,  by  the  suction  of  the  column 
of  liquid,  the  formation  of  which  is  established  by  the  liquid  col- 
lecting in  the  bend.1  The  narrow  tube  must  not  be  attached  to  the 
stem  by  a  bit  of  rubber  tubing,  because  of  the  liability  to  loss  or 
contamination  of  the  filtrate  through  creeping  of  liquid  between  the 
rubber  and  glass. 

A  small  funnel,  4  cm.  in  diameter  and  with  straight  stem  about 
7  cm.  long,  is  needed  for  small  filtrations;  and  another  5  or  6  cm. 
in  diameter,  with  the  stem  cut  off  half-way,  slightly  drawn  out, 
and  bent  at  about  120°,  is  used  for  introducing  the  hydrochloric 
acid  into  the  platinum  basin  (p.  137).  It  is  well  to  have  on  hand 
several  other  ordinary  funnels,  of  various  sizes.. 

Gas  Generators.— Kipp's  form  (E.  &  A.,  2674,  A.  T.  Co., 
29,572)  is  about  the  best.  There  should  be  one  each,  of  medium 
size,  for  carbon  dioxide  and  hydrogen  sulphide. 

Gas  Washing  Cylinders. — A  good  type  is  Muencke's  (E.  &  A., 
1044,  A.  T.  Co.,  29,472),  and  another  about  equally  good  is 
Drexel's  tall  form  (E.  &  A.,  1036,  A.  T.  Co.,  29,452) .  There  should 
be  two  or  three  of  medium  sizes. 

1  See  Ostwald,  p.  14.  It  is  somewhat  surprising  that  such  funnels,  which 
have  been  in  common  use  for  many  years,  are  not  listed  in  the  dealers'  cata- 
logues. 


APPARATUS  37 

Glass  Cells  (E.  &  A.,  6334),  with  parallel  sides,  for  colorimetric 
determinations  (p.  43).  A  pair  with  identical  internal  distance 
between  parallel  sides  will  be  needed. 

Measuring  Cylinders. — These  should  be  lipped  and  not  stop- 
pered. They  should  be  reasonably  accurate,  but  it  is  not  neces- 
sary that  they  be  "  certified."  There  should  be  one  each  of  10, 
25,  100,  and  500  c.c. 

Measuring  Flasks. — These  should  all  be  stoppered,  and  it  is 
best  to  purchase  them  "  certified,"  as  these  will  need  no  calibra- 
tion for  ordinary,  good  work.  One  each  of  1000  and  500  c.c., 
and  three  each  of  250,  200,  and  100  c.c.  will  suffice. 

Pipettes. — Two  each,  of  5  and  10  cc.  and  one  of  50  c.c.,  with  the 
bulb  in  the  middle  of  the  stem,  are  sufficient.  Their  capacity 
should  be  indicated  by  marks  above  and  below  the  bulb. 

Separatory  Funnel. — One  pear-shaped,  separatory  funnel,  with 
glass  stopcock  and  stopper,  and  rather  wide  stem,  may  be  needed 
occasionally  for  mineral  separations  with  heavy  solutions.  A 
capacity  of  100  c.c.  is  about  the  right  size. 

Specimen  Tubes. — There  should  be  a  supply  of  several  dozen 
of  these,  with  round  bottoms,  and  with  smooth  corks  to  fit.  Appro- 
priate sizes  are:  4X^,  4Xf,  and  5X|  inches,  of  which  the  first 
will  be  the  most  generally  used. 

Stirring  Rods. — The  diameter  of  these  should  not  be  over 
3  mm. ;  it  is  common  with  beginners  to  use  too  thick  stirring  rods. 
About  twenty  should  be  prepared,  of  various  lengths,  from  10  to 
22  cm.  long;  the  shorter  stirrers  being  made  of  the  thinner  rod. 
Both  ends  should  be  rounded,  and  about  2  cm.  of  one  end  bent 
back  sharply  at  about  60°.  It  is  not  well  to  use  stirring  rods  that 
are  alike  at  both  ends,  because  of  the  danger  of  mistaking  the 
immersed  part  (with  adherent  solution  or  precipitate)  for  the 
clean  end. 

One  of  the  long  and  one  of  the  medium  stirrers  should  be  pro- 
vided with  a  rubber  " policeman."  The  narrow  shape  (E.  &  A., 
6032,  A.  T.  Co.,  46,164)  is  about  the  best  for  general  use.  An 
exellent  one  may  be  made  1  from  a  No.  00  (10  mm.)  solid  rubber 
stopper,  by  boring  a  small  hole  half  way  through  axially  from  the 
small  end,  removing  the  core,  and  cutting  and  grinding  down  the 
larger  end  to  a  smooth  wedge. 

1  Suggested  by  Dr.  J.  C.  Hostetter. 


38  APPARATUS  AND  REAGENTS 

Test  Tubes. — A  few  each  of  different  sizes,  from  small  to 
medium  (say  10X1.2  to  16X2  cm.)  should  be  kept. 

Thermometer. — A  thermometer,  reading  to  150°  at  least,  and 
preferably  to  250°,  is  needed  for  the  hot-air  oven.  This  may  have 
an  opal  glass  scale.  It  will  also  be  well  to  have  one  or  two  more, 
with  scale  etched  on  the  stem,  one  reading  to  300°,  for  various 
purposes. 

Tubing. — One  will  need  sufficient  tubing  of  rather  soft  glass 
and  of  various  diameters  for  making  connections,  etc.  There 
should  also  be  a  supply  of  special  glass  tubing,  of  an  internal 
diameter  of  5  or  6  mm.,  for  the  determination  of  water.  This 
tubing  may  be  either  thick-walled  and  of  soft  glass,  or  (better) 
thin-walled  and  of  refractory  glass,  but  not  as  refractory  as  com- 
bustion tubing.  The  glass  must  not  devitrify  on  heating,  as  often 
happens  with  old  glass. 

Wash-bottles. — Two  main  wash-bottles  are  needed.  These 
should  be  of  resistance  glass,  flat-bottomed,  and  of  1000  c.c. 
capacity.  The  tip,  with  an  exit  hole  1  mm.  in  diameter,  is  at- 
tached by  a  short  length  of  rubber  tubing.  About  5  cm.  of  the 
lower  end  of  the  exit  tube  should  be  bent  at  about  120°,  so  that 
the  intake  is  near  the  lower  side  wall,  and  in  the  same  plane  and 
direction  as  the  tip.  This  enables  more  of  the  liquid  to  be  expelled 
than  if  the  tube  is  straight.  One  of  these  wash-bottles,  for  hot 
water,  has  its  neck  protected  against  the  heat.  This  may  be  done 
with  asbestos  paper,  which  is  wrapped  around  in  several  thick- 
nesses and  moistened,  when  it  adheres;  or  with  a  thin  sheet  of 
cork,  bent  entirely  round  the  neck  and  held  in  place  by  copper 
wire.  The  latter  is  preferable,  as  the  asbestos  paper  is  liable  to 
flake  off  and  contaminate  tfce  material  in  the  process  of  analysis. 

There  should  also  be  a  wash-bottle  (500  c.c.)  reserved  for  the 
dilute  ammonia  water  used  in  washing  the  magnesia  precipitate, 
and  another  (300  c.c.)  reserved  for  the  alcohol  used  in  the  alkali 
determination.  It  is  best  to  have  these  with  glass  stoppers 
(E.  &  A.,  1132,  A.  T.  Co.,  48,960)  as  ammonia  and  alcohol  attack 
rubber,  and  it  is  well  to  label  each  specially.  A  spare,  300  c.c. 
wash-bottle  will  be  convenient  for  various  washing  solutions. 

Watch-glasses. — Six  each  of  2,  2J,  3,  3J,  4,  and  5  inches,  and 
three  each  of  6,  6J,  and  7  inches,  should  be  on  hand.  It  will  be 
convenient  to  have  one  each  of  the  4-,  5-,  and  6-inch  glasses  per- 


APPARATUS  39 

forated  with  a  central  hole  about  5  mm.  in  diameter.  A  watch- 
glass  is  always  to  be  placed  on  a  beaker  with  the  convex  side  down. 
A  pair  of  the  2J-  or,  better  the  3-inch  glasses,  weighing  very 
nearly  alike,  should  be  adjusted  to  equal  weight  by  filing  the  edge 
of  the  heavier.  These  are  used  in  weighing  out  fluxes,  and  are 
to  be  marked  with  a  diamond  and  kept  in  the  balance  case. 

FUSED    SILICA 

Because  of  its  high  melting-point,  resistance  to  all  acids  (except 
hydrofluoric),  constancy  in  weight  under  ignition,  and  low  coeffi- 
cient of  expansion,  fused  silica,  which  can  now  be  obtained  of 
very  good  quality  and  in  a  great  variety  of  forms,  may  replace 
platinum  for  some  purposes.  In  selecting  utensils  made  of  it, 
care  must  be  taken  to  see  that  the  interior  surface  is  "  flashed," 
that  is,  rendered  quite  smooth  by  intense  heating,  so  that  no 
bubble  cavities  are  open  to  the  interior  surface.  If  the  interior 
surface  is  thus  made  smooth  it  is  not  necessary  that  the  material 
be  of  the  more  expensive,  transparent  variety.  Strongly  alkaline 
liquids  should  not  be  heated  for  long  in  fused  silica  vessels. 

Basin. — This  must  be  lipped.  One  about  13  cm.  in  diameter 
and  7  cm.  deep,  with  a  capacity  of  about  500  c.c.,  will  serve  for 
the  alkali  determination. 

Beaker. — It  is  well,  but  not  necessary,  to  have  a  fused  silica 
beaker  for  the  alkali  determination.  This  is  best  of  400  c.c. 
capacity,  and  lipped. 

Triangles. — Instead  of  platinum  or  palau  triangles,  those  made 
of  fused  silica  may  be  used  with  advantage,  as  they  are  much 
cheaper  and  there  is  no  liability  of  the  crucible  sticking  to  the 
triangle.  Those  made  of  tubing  strung  on  nickel  or  nichrome  wire 
are  excellent. 

POKCELAIN 

There  are  several  very  good  makes  of  American,  as  well  as 
Japanese  and  Copenhagen,  porcelain  on  the  market,  which  are 
quite  equal  to,  and  very  satisfactory  substitutes  for,  Royal  Berlin 
and  other  German  makes.  The  porcelain  should  be  glazed  inside 
and  out. 

Casseroles. — These  should  be  lipped  and  with  porcelain  han- 


40  APPARATUS  AND  REAGENTS 

dies.  Covers  are  not  needed.  One  or  two  each  of  165  and  135 
mm.,  with  possibly  a  smaller  one,  will  be  enough. 

Crucibles. — Two  or  three  of  different  sizes,  with  covers,  are 
useful.  One  about  5  cm.  in  diameter  may  be  reserved  as  a  "  radi- 
ator "  for  the  evaporation  of  sulphuric  acid  in  platinum  crucibles, 
but  a  nickel  crucible  is  better  (p.  145).  A  porcelain  Gooch 
crucible,  35  or  40  mm.  in  diameter,  may  replace  one  of  platinum. 
It  is  well  to  have  with  this  a  circular,  perforated  disc  of  porcelain. 

Evaporating  Dishes. — Two  or  three  each  of  9-,  10-,  12-  and 
15-cm.  lipped,  should  be  on  hand. 

Plate. — A  square,  white,  porcelain  plate,  12  or  15  cm.  in  diam- 
eter, is  useful  for  titrations,  as  well  as  for  the  separation  of  min- 
erals. 

RUBBER 

Funnel. — A  hard  rubber  funnel,  of  the  smallest  size,  is  needed 
for  filtration  of  liquids  containing  hydrofluoric  acid,  if  one  of  plat- 
inum is  not  available. 

Stoppers. — There  should  be  a  selection  of  several  sizes,  per- 
forated with  one  and  two  holes. 

Tubing. — A  variety  of  different  sizes,  for  making  connections, 
should  be  available.  There  should  also  be  about  1  foot  of  black, 
soft,  thin-walled  tubing  for  Gooch  crucibles  (E.  &  A.,  6064,  A.  T. 
Co.,  46,236),  of  a  diameter  appropriate  to  the  crucible  in  use. 

METAL 

Burners. — Three  or  four  Bunsen  burners  should  be  provided. 
The  Tirrill  type  (E.  &  A.,  1462,  A.  T.  Co.,  22,884)  is  very  con- 
venient, as  it  permits  easy  adjustment  of  both  gas  and  air.  One 
Meker  burner  (E.  &  A.,  1468,  A.  T.  Co.,  22,916),  of  medium  size, 
is  useful  for  ignitions  at  higher  temperatures. 

There  should  also  be  available  a  blast  Bunsen  burner  of  the 
usual  type. 

Clamps. — There  should  be  on  hand  several  clamps  of  different 
types  and  sizes.  A  large-sized  one  of  the  "  universal  "  type  (E.  & 
A.,  2034,  A.  T.  Co.,  24,508),  with  rubber-covered  jaws,  is  used  to 
hold  the  weighing  burette.  There  should  be  also  several  clamp 
fasteners. 


APPARATUS  41 

Hot  Plate. — An  electric  hot  plate  with  three  "heats"  is  very 
useful. 

Igniter. — An  igniter,  made  of  cerium-iron  alloy,  which  can 
be  obtained  at  any  hardware  store,  is  useful  and  preferable  to 
matches. 

Mortar. — A  hardened  steel  mortar  is  necessary  for  the  crushing 
of  the  rock  specimen.  It  is  imperative  that  the  steel  be  as  hard  as 
possible,  to  avoid  contamination  by  iron.  The  mortar  should 
also  be  readily  taken  apart  and  freed  from  all  the  rock  powder, 
and  easily  cleaned.  It  should  also  be  sufficiently  large  to  take  in  a 
rock  fragment  2  or  3  cm.  in  greatest  dimension. 

The  best  known  to  me  is  the  type  devised  and  made  by  C.  W.  H. 
Ellis,  mechanician  in  the  Geophysical  Laboratory  of  the  Carnegie 
Institution  of  Washington.  These  are  made  of  a  "  special  tool  " 
steel,  hardened  by  Mr.  Ellis,  by  a  treatment  of  his  own.1  They 
are  made  in  different  sizes.  One  of  the  largest,  used  in  the  U.  S. 
Geological  Survey  laboratory,  is  figured  by  Hillebrand,2  with  its 
dimensions  given. 

These  mortars  are  of  the  Plattner  style,  in  three  pieces,  base, 
cylinder,  and  pestle,  but  the  cylinder  is  very  high,  to  guard  against 
loss  of  fragments  and  dust.  They  are  all  larger  than  the  Plattner 
"  diamond  "  mortars  ordinarily  obtainable.  The  pestle  may  be 
of  two  forms;  one  with  a  large  knob  at  the  upper  end  (as  figured  by 
Hillebrand),  for  use  with  the  hand  alone,  the  pestle  acting  as 
hammer;  and  the  other  slightly  tapering  toward  the  top,  for  use 
with  a  hammer.  The  base  may  be  square  or  round.  The  bottom 
of  the  cavity  in  the  base  is  flat,  the  cylinder  fitting  snugly  into  the 
cavity,  while  the  pestle  has  a  little  play  in  the  cylinder.  The 
dimensions  of  the  one  I  use,  which  is  of  the  knobless  form,  are  as 
follows:  Base,  6  cm.  square,  2.5  cm.  thick;  cavity,  3.2  cm.  diam- 
eter, 0.5  cm.  deep;  cylinder,  8  cm.  high,  3.2  (scant)  external  and 
2.6  internal  diameter;  pestle,  11.5  cm.  long,  2.5  cm.  diameter  at 
base,  2.4  at  top. 

If  one  of  these  is  not  procurable,  a  first-quality,  three-piece, 
"  diamond,"  Plattner's  mortar  may  be  used.  It  is  important 
that  the  steel  be  as  hard  as  possible,  a  point  to  which  especial 
attention  should  be  paid,  that  the  pestle  fit  somewhat  loosely  in 

1  The  hardness  of  mine  is  8. 

2  Hillebrand,  Bull.  422,  p.  51. 


42  APPARATUS  AND  REAGENTS 

the  cylinder,  and  that  the  bottom  of  the  cavity  be  flat.1  Those 
usually  listed  in  the  dealers'  catalogues  (E.  &  A.,  4626,  A.  T. 
Co.,  40,808)  are  intended  for  blowpiping,  and  are  too  small 
for  convenient  use  with  rock  specimens,  though  they  will 
serve. 

The  steel  mortar  should  be  exposed  as  little  as  possible  to  the 
fume-laden  atmosphere  of  the  laboratory,  and  ought  to  be  kept  in 
a  tightly  closed  box,  to  prevent  rusting.  It  should  not  be  han- 
dled with  moist  fingers,  and  after  use  should  always  be  immediately 
wiped  dry  with  a  clean  and  dry  cloth  and  placed  in  its  box.  It 
should  never  be  "  left  around  "  exposed  to  the  air  and,  of  course, 
must  not  be  greased  or  oiled,  however  lightly. 

Nickel  Crucible. — A  nickel  crucible,  used  as  a  radiator,  is 
better  than  one  of  porcelain  for  the  evaporation  of  sulphuric  acid 
(p.  145),  as  the  operation  is  more  quickly  carried  out.  One  5  cm. 
in  diameter,  4  cm.  high,  and  of  about  60  c.c.  capacity,  is  of  con- 
venient size, 

.  Oven. — A  copper  oven  with  single  walls  is  needed.  A  con- 
venient size  is  8X6X6  inches.  One  shelf  is  sufficient.  It  should 
be  provided  with  legs  or  a  suitable  tripod  support. 

Retort  Stands. — At  least  three,  and  preferably  four,  should  be 
on  hand.  The  base  should  be  rectangular — not  triangular,  as 
these  are  very  unstable.  The  rod  may  be  16  or  18  inches  high. 
Rings,  with  clamp  attached,  3  inches  in  diameter  are  most  useful, 
but  it  is  well  to  have  also  a  4-inch  ring.  It  is  well  to  have  one  of 
the  3-inch  rings  with  three  interior  knobs  (A.  T.  Co.,  46,072)  for 
supporting  the  porcelain  crucible  in  evaporating  sulphuric  acid. 
It  is  unwise  to  have  more  than  one  ring  on  the  stand  at  a  time. 
The  rod  may  advantageously  be  covered  with  a  piece  of  glass 
tubing  closed  at  the  top,  to  prevent  the  falling  of  rust.  The 
retort  stands  and  rings  should  be  wiped  off  frequently.  There  is 
constant  danger  of  contamination  by  iron  rust  from  them,  and 
it  would  be  well  if  some  made  of  nickel,  nichrome,  or  some  such 
non-rusting  metal  were  on  the  market. 

Water  Baths. — If  the  laboratory  is  not  provided  with  a  steam 
bath,  two  water  baths  should  be  on  hand.  These  are  to  be  of 

1  The  recommendation  of  a  hemispherical  cavity  (first  edition,  p.  51), 
applied  only  to  the  two-piece,  Leed's  form.  It  was  corrected  in  the  second 
edition,  p.  54. 


APPARATUS  43 

copper,  preferably  with  porcelain  rings,  and  fitted  with  a  Kekule 
regulator.     (E.  &  A.,  628,  A.  T.  Co.,  49,048.) 

Wire  Gauze. — Two  squares  each,  of  4  and  5  inches,  are  needed. 
These  are  best  made  of  nichrome  wire,  as  they  do  not  rust  and 
long  outlive  those  of  iron.  It  is  well  to  have  one  with  an  asbestos- 
covered  center  for  heating  the  platinum  basin. 

MISCELLANEOUS 

Agate  Mortar. — An  agate  mortar,  about  9  cm.  in  outside  diam- 
eter, is  needed  for  the  preparation  of  the  rock  specimen.  One 
should  be  selected  that  has  no  soft  spots  or  rough  streaks,  at  least 
in  the  interior.  The  pestle  should  be  embedded  firmly 1  in  a 
wooden  handle  with  cement  and  a  brass  collar,  making  the  whole 
about  2J  inches  long.  It  will  be  useful,  but  not  necessary,  to 
have  another  larger  agate  mortar,  about  13  cm.  in  diameter.  The 
pestle  of  this  needs  no  handle.  Substances  should  only  be  ground, 
never  pounded,  in  an  agate  mortar. 

Burette  Stand. — One  for  two  burettes  is  needed.  A  very 
convenient  type  is  the  Chaddock  (E.  &  A.,  6548,  A.  T.  Co.,  22,692) 
with  rubber-covered  wire  spring  clamps  and  white  glass  base. 

Colorimeter. — A  colorimeter  is  needed  for  the  colorimetric 
determination  of  titanium  dioxide,  manganous  oxide,  and  chromic 
oxide.  The  simple  form  described  by  Hillebrand2  is  very  satis- 
factory and  serves  all  ordinary  purposes.  The  more  elaborate 
forms  devised  by  Schreiner  (E.  &  A.,  2116,  A.  T.  Co.,  24,738) 
and  Steiger  3  are  excellent  but  much  more  complex  and  expensive. 

The  essential  part  of  the  simple  form,  whose  use  is  assumed  in 
the  operations  described  later,  is  a  pair  of  glasses  or  glass  cells,  of 
square  or  rectangular  section.  Two  opposite  sides  of  each  must  be 
parallel,  and  the  interior  distances  apart  of  these  sides  in  the  two 
glasses  must  be  identical,  at  most  within  1  per  cent  of  the  distance. 
The  other  sides  need  not,  but  would  best  be,  parallel,  and  may  be 
blackened  to  exclude  light.  The  glasses  used  by  me  are  12  cm. 
high,  and  4  cm.  between  the  parallel  sides  each  way. 

These  glasses  are  made  of  thin  (2-3  mm.)  plate  glass,  cut  to 

1  Mine  has  been  in  use  for  over  twenty  years  and  is  still  firm. 

2  Killebrand,  p.  33;  Mellor,  p.  84. 

3  Hillebrand,  pp.  37,  35. 


44  APPARATUS  AND  REAGENTS 

size,  and  cemented  with  a  cement  that  will  resist  the  action  of  dilute 
acids.  The  angles  may  be  strengthened  by  rubber  tape. 

The  use  of  a  suitable  box  is  necessary  to  exclude  side  lights,  and 
that  illustrated  by  Hillebrand  serves  well.  This  may  be  made 
easily  from  a  box  in  which  are  packed  the  ceresine  bottles  contain- 
ing one-half  pound  of  ammonia  water  or  hydrofluoric  acid. 

The  box  measures  20X9.5X9.5  cm.  internally.  The  square 
bottom  is  removed,  leaving  the  box  open  at  either  end.  A  3-inch 
square  of  ground  glass  is  substituted  for  the  sliding  cover,  the  glass 
slipping  snugly  into  the  grooves,  which  may  need  a  little  widening 
with  a  penknife.  About  5  cm.  of  the  side  next  to  the  free  edge 
of  the  glass  is  then  cut  away,  to  allow  the  insertion  of  the  pair  of 
glasses.  This  side  now  becomes  the  top  of  the  box.  A  thin  wooden 
partition  (made  of  the  cover  of  the  box)  is  inserted  through  a 
narrow  slot  cut  clear  across  the  top  of  the  box  alongside  the  glasses 
away  from  the  ground  glass.  This  shutter  should  slide  stiffly  up 
and  down,  so  as  to  remain  at  any  desired  height,  or  a  thin  wedge 
may  be  used  to  hold  it  in  place.  The  box  and  partition  are  black- 
ened inside  and  out,  and  the  result  is  a  box  that  is  light  and  com- 
pact enough  to  be  held  easily  in  the  hand. 

Filter  Papers. — Round-cut  filters  are  to  be  used,  the  paper 
being  of  such  quality  as  to  leave  but  a  negligible  amount  of  ash, 
filter  fairly  rapidly,  and  yet  retain  the  finest  precipitates.  They 
must  be  "  double- washed  "  with  hydrochloric  and  hydrofluoric 
acids,  of  the  type  of  the  Schleicher  and  Schiill  No.  590.  Baker 
and  Adamson,  "  A  "  quality,  is  excellent,  and  some  of  the  What- 
man paper  will  serve.  The  sizes  in  regular  use  are :  5  J-,  7-,  9-,  and 
11-cm.,  of  which  there  should  be  two  packages  each  and  one  of 
12J  cm.  The  filters  are  to  be  kept  in  a  drawer  away  from 
dust. 

Funnel  Supports.  Two  wooden  funnel  supports  should  be 
sufficient.  My  preference  is  for  the  single-arm  type  for  two  fun- 
nels, with  one  hole  larger  than  the  other.  The  two-arm  type 
takes  up  (often)  unnecessary  space,  and  the  two  arms  cannot  be 
differently  adjusted  as  to  height. 

Horn  Spoon. — One  about  12  cm.  long  is  used  for  weighing  out 
fluxes.  It  may  conveniently  be  kept  in  the  balance  case. 

Labels. — A  box  of  medium  small  labels,  such  as  Dennison's 
No.  217,  should  be  on  hand. 


REAGENTS  45 

Sieve. — A  sieve  l  for  preparing  the  rock  powder  is  made  of  a 
cylindrical  glass  box,  about  8  cm.  diameter  and  4J  cm.  high. 
Closely  fitting  this  is  a  brass  ring,  about  1  cm.  wide,  which  holds 
in  place  a  4-inch  square  of  silk  bolting  cloth.  To  prevent  the 
loss  of  dust  (p.  69)  the  brass  ring  may  be  2  cm.  wide  and  a  second 
glass  box  of  the  same  diameter  as  the  first  fitting  into  the  upper 
half  of  the  ring,  acts  as  a  receptacle  for  the  rock  powder. 

Silk  Bolting  Cloth. — That  which  is  best  for  preparing  the  sam- 
ple is  of  100  mesh  (40  meshes  to  the  centimeter).  Three  square 
feet  of  this  had  best  be  kept.  Some  of  50-mesh  will  be  useful  for 
mixing  the  sample  (p.  71). 

Stone  Slab. — A  useful  adjunct  is  a  small  slab  of  some  igneous 
rock,  such  as  granite,  on  which  to  cool  crucibles.  This  may  be 
4X3X1  inches,  and  is  polished  on  the  top  face.  A  steel  block  of 
about  the  same  size  may  be  used,  but  should  be  nickel-  or  gold- 
plated  to  prevent  rusting. 

Support. — A  Gay-Lussac  or  Schellbach  "  universal,"  wooden 
support  will  often  be  found  useful. 

Test-tube  Rack. — One  of  wood  for  six  tubes,  with  draining 
pegs,  should  be  sufficient. 

2.  REAGENTS 

The  matter  of  the  quality  of  reagents  2  is  most  important,  and 
one  that  is  far  too  often  neglected.  Only  those  of  the  very  best 
quality  should  be  used,  and  these  cannot  always  be  taken  on  faith. 
It  is  a  good  precaution  to  test  even  the  so-called  "  guaranteed  " 
or  "  analyzed  "  reagents  for  impurities,  by  the  methods  suggested 
by  Krauch,3  Merck,4  and  Fresenius  5  and  in  some  cases  for  impur- 
ities not  considered  by  them.  This  is  because,  while  they  are  gen- 
erally satisfactory,  yet  mistakes  will  occur  and  it  is  very  difficult 
to  free  some  reagents  from  certain  impurities.  As  Hillebrand  says: 

1  Sieves  such  as  these  are  useful  for  many  separations  and  they  might  well 
be  listed  in  the  dealers'  catalogues. 

2  Cf.  Fresenius,  1,  p.  49;  Hillebrand,  Bull.  422,  p.  39;  Mellor,  p.  141. 

3  C.  Krauch,  Trans,  by  Williamson  and  Dupre,  The  Testing  of  Chemical 
Reagents  for  Purity.     1902. 

4  E.  Merck,  Trans,  by  H.  Schenck,  Chemical  Reagents,  their  Purity  and 
Tests.     1914. 

5  Fresenius,  Qual.  Anal.,  pp.  52  ff.,  Quant.  Anal.,  1,  pp.  127  ff. 


46  APPARATUS  AND  REAGENTS 

"A  C.  P.  label  is  no  guarantee  of  the  purity  of  a  reagent,"  and 
samples  of  "  guaranteed  "  reagents  have  been  found  to  be  worse 
than  some  without  any  special  guarantee.1 

In  recent  years  there  has  been  a  great  improvement,  and  the 
"  guaranteed  reagents  "  furnished  by  such  makers  as  Baker  and 
Adamson,  The  J.  T.  Baker  Chemical  Co.,  and  Squibb  are  gener- 
ally excellent. 

It  is  well  to  stock  up  with  amounts  of  reagents  (except  the 
strong  acids  and  ammonia  water),  sufficient  to  last  for  a  very  con- 
siderable time,  and  to  test  each  new  lot  once  for  all.  It  must  be 
said,  however,  that  the  "  guaranteed  "  concentrated  hydrochloric, 
nitric,  and  sulphuric  acids,  and  hydrofluoric  acid  (in  ceresine  bot- 
tles), are  nearly  always  reliable  and  scarcely  need  special  testing, 
except  that  nitric  acid  is  to  be  examined  for  chlorine. 

All  the  reagents  in  use,  on  the  work-bench  shelves,  are  to  be 
kept  in  glass-stoppered  bottles2  of  good  quality;  the  narrow- 
mouth  ones  (best  with  a  vertical  flat  stopper)  are  kept  covered  with 
loose-fitting  glass  caps,  and  the  wide-mouth  ones  should  have  a 
flat  stopper  projecting  over  the  lip,  to  keep  off  dust.  It  is  best, 
when  possible,  to  have  the  labels  either  etched  or  ground  on  the 
glass  (E.  &  A.,  958,  976,  A.  T.  Co.,  22,270,  22,304),  or,  better, 
enamelled  (E.  &  A.,  950,  A.  T.  Co.,  22  328).  Paper  labels  should 
be  varnished.  The  strength  of  standard  solutions  should  be  writ- 
ten on  the  label. 

It  may  be  convenient  to  give  a  list  of  the  different  sizes  of 
reagent  bottles  appropriate  for  the  reagents  in  most  general  use. 
Most  of  these  can,  and  all  ought  to  be  obtainable  (of  the  dealers) 
in  these  sizes  with  permanent  labels  as  mentioned  above. 

Narrow-mouth  Bottles,  500  c.c. — Alcohol,  ammonium  hydrox- 
ide,3 ammonium  molybdate,  ammonium  nitrate,  hydrochloric 
acid,  nitric  acid,  potassium  chromate,  standard  manganese  solu- 
tion, silver  nitrate,  standard  titanium  solution,  sulphuric  acid 
(cone.),  sulphuric  acid  (1:1). 

'  1  In  this  laboratory  "  guaranteed  "  ammonia  water  has  been  found  to  con- 
tain zinc;  potassium  chromate,  crystals  of  potassium  nitrate;  ammonium 
acetate,  much  cadmium;  and  magnesia,  about  3  per  cent  of  lime. 

2  Cf.  Mellor,  p.  142. 

3  The  bottle  should  be  coated  internally  with  ceresine  or  paraffine,  or  a 
ceresine  bottle  is  used  in  place  of  glass.     See  p.  48. 


REAGENTS  47 

250  c.c. — Acetic  acid,  barium  chloride,  chloroplatinic  acid, 
ether,  hydrogen  peroxide,  magnesia  mixture,  potassium  thiocya- 
nate. 

Wide-mouth  bottles,  500  c.c. — Ammonium  carbonate,  ammo- 
nium chloride,  ammonium  oxalate,  calcium  carbonate,  ferrous  sul- 
phide, sodium  ammonium  phosphate,  sodium  carbonate,  potas- 
sium pyrosulphate. 

250  c.c. — Ammonium  nitrate,  ammonium  persulphate,  asbestos, 
sodium  acetate,  potassium  nitrate. 

In  the  following  list  of  reagents  (with  the  exceptions  of  alcohol, 
ammonia  water,  hydrochloric,  hydrofluoric,  nitric,  and  sulphuric 
acids),  amounts  are  suggested  that  should  be  sufficient  to  last 
several  years,  in  ordinary  analytical  work.  They  should  all  be 
"  guaranteed  reagents  "  with  the  exception  of  a  few  that  are  noted. 

Acid,  Acetic  (250  grams). — Acid  of  30  per  cent  strength  will 
answer  for  use  in  the  sodium  acetate  method. 

Acid,  Hydrochloric  l  (6  pounds). 

Acid,  Hydrofluoric  (2-8-ounce  bottles). — This  must  be  obtained 
and  kept  in  ceresine  bottles  (never  in  rubber  or  gutta  percha). 
The  greatest  possible  caution  should  be  observed  in  its  use,  as  a 
sore  produced  by  it  may  last  for  years. 

Acid,  Nitric  (7  pounds). 

Acid,  Sulphuric  (9  pounds). 

Alcohol,  Ethyl,  Absolute  (8  ounces). — This  is  only  used  in  the 
determination  of  strontium.  The  stopper  should  be  very  close- 
fitting. 

Alcohol,  Ethyl,  "95  Per  Cent"  (2  quarts) .—This  should  be  of 
good  quality,  not  denatured,  and  it  is  well  to  filter  it.  For  use  in 
the  determination  of  alkalies  it  is  diluted  with  distilled  water  to  a 
specific  gravity  of  0.86.  If  a  hydrometer  is  not  available,  this 
specific  gravity  may  be  attained  approximately  by  mixing  five 
volumes  of  alcohol  with  one  of  water.  The  liquids  are  to  be 
measured  out  separately,  because  of  the  contraction  that  takes 
place  on  mixing. 

Ammonium  Carbonate  (1  pound). — This  should  contain  no 
non-volatile  matter  and  only  a  trace  of  tarry  matter.  The  solu- 
tion is  made  as  needed. 

1  Commercial  hydrochloric  acid  will  serve  for  generating  carbon  dioxide 
and  hydrogen  sulphide.  The  strength  should  be  1  :  1. 


48  APPARATUS  AND  REAGENTS 

Ammonium  Chloride  (1  pound). — This  is  not  likely  to  contain 
impurities  that  render  it  unfit  for  the  alkali  determination,  for 
which  alone  it  is  used,  but  it  should  be  tested  to  see  that  it  con- 
tains no  non-volatile  matter.  Re-sublimation  is  seldom  called  for. 
It  should  not  be  used  for  the  addition  of  ammonium  chloride  in 
the  main  portion  of  the  analysis,  where  it  is  formed  instead  by 
the  neutralization  of  hydrochloric  acid  by  ammonia. 

Ammonium  Hydroxide  (4  pounds). — This  must  leave  no  solid 
residue  on  standing  or  on  evaporation,  and  it  must  be  free  from 
carbonate.  Immediately  before  use  a  small  portion  should  be 
tested  with  calcium  or  barium  chloride.  If  ammonium  carbonate 
is  present  the  ammonia  water  is  to  be  boiled  to  decompose  the 
carbonate. 

Ammonia  water  ("  guaranteed  ")  may  be  purchased  in  ceresine 
bottles,  but  its  delivery  in  this  material  is  not  a  guaranty  that  it 
has  not  been  kept  in  glass.  It  is  best  to  make  it  from  time  to  time 
by  passing  ammonia  gas  from  a  cylinder  through  (previously 
boiled  and  cooled)  distilled  water  contained  in  a  ceresine  bottle. 
This  should  be  only  half  full  and  surrounded  with  water  and 
cracked  ice.  The  gas  should  be  passed  until  the  specific  gravity  is 
0.92.  The  purity  of  the  reagent  thus  prepared  will  compensate 
for  the  trouble  involved. 

Ammonia  water,  intended  for  analytical  work,  should  never 
be  kept  in  glass,  as  this  is  inevitably  attacked  more  or  less,  render- 
ing the  reagent  impure.  For  the  best  work,  indeed,  glass  vessels 
should  not  be  used  for  precipitations,  etc.,  in  ammoniacal  liquids.1 

The  reagent  is  best  kept  in  ceresine  bottles,  with  a  tight-fitting 
ceresine  stopper  that  is  closed  by  a  screw  motion,  and  covered  with 
a  loose-fitting  glass  cap.  It  may  be  kept  in  a  glass  bottle  coated 
all  over  the  interior  with  ceresine,  by  putting  a  few  pieces  into  the 
perfectly  dry  bottle,  gently  warming  the  bottle,  and  spreading  the 
wax  by  turning  the  bottle  around.  This  coating,  however,  is  likely 
to  loosen  in  time  and  allow  the  ammonia  to  attack  the  glass,  so 
that  a  ceresine  bottle  is  preferable. 

Ammonium  Molybdate  (1  pound). — This  salt  is  usually  suf- 
ficiently pure  for  use  without  testing.     The  solution  used  in  the 
determination  of  phosphorus  pentoxide  is  prepared  by  dissolving 
50  grams  of  ammonium  molybdate  in  250  c.c.  of  water,  with  the 
1  Cf.  Allen  and  Johnston,  Jour.  Ind.  Chem.  Eng.,  2,  p.  202,  1910. 


REAGENTS  49 

aid  of  heat,  and  pouring  it  when  cold  into  250  c.c.  of  concentrated 
nitric  acid,  with  constant  stirring.  The  precipitate  that  first 
forms  redissolves  on  addition  of  all  the  ammonium  molybdate. 
The  mixture  is  filtered  through  an  asbestos  plug  in  a  funnel,  after 
standing  a  few  days. 

Ammonium  Nitrate  (8  ounces). — A  solution  containing  340 
grams  of  this  to  the  liter  is  needed  in  the  determination  of  phos- 
phorus pentoxide,  and  the  salt  is  occasionally  used  in  the  solid 
form. 

Ammonium  Oxalate  (1  pound). — It  is  well  to  recrystallize  this, 
after  filtering,  to  insure  its  freedom  from  calcium  oxalate.  The 
solution  is  made  as  needed. 

Ammonium  Persulphate  (8  ounces). — The  ordinarily  pure  salt 
will  answer  for  the  colorimetric  determination  of  manganese,  but 
for  the  precipitation  of  manganese  with  alumina  the  "  guaran- 
teed "  salt  should  be  specially  tested,  or  purified  by  the  method 
recommended  by  Hillebrand.1  The  salt  is  used  in  the  solid 
form. 

Asbestos  (4  ounces). — This  should  be  the  "  true  "  asbestos,  that 
is,  the  amphibole  variety,  not  the  chrysotile  or  serpentine  variety, 
which  also  goes  under  the  name  of  asbestos.2  The  former  is  prac- 
tically anhydrous  and  insoluble  in  acids,  while  the  latter  is  par- 
tially soluble  in  acids  and  contains  about  13  per  cent  of  water, 
which  is  lost  on  ignition,  resulting  in  the  breaking  down  of  the 
fibrous  texture. 

A  good  quality,  perfectly  white,  silky,  but  not  necessarily  very 
long  fiber,  should  be  selected.  About  10  or  20  grams  is  macerated 
with  water  in  a  porcelain  mortar,  or  if  of  long  fiber  is  scraped  down 
with  a  knife  and  macerated  with  water.  The  mass  is  placed  in  a 
400  c.c.  beaker,  200  c.c.  of  water  and  20  c.c.  of  hydrochloric  acid 
are  added;  the  mixture  is  boiled  for  half  an  hour,  and  allowed  to 
stand  over  night  on  the  steam  bath.  It  is  then  well  washed  with 
distilled  water,  and  preferably  allowed  again  to  stand  over  night 
with  dilute  acid  to  remove  all  the  soluble  iron.  Finally  it  is  thor- 
oughly washed  with  hot  water  on  a  large  filter  (best  with  suction) 

1  Hillebrand,  Bull.  422,  p.  102. 

2  Cf.  Dana,  System  of  Mineralogy,  pp.  389,  670,  1892;  F.  Cirkel,  Chryso- 
tile-Asbestos,  Ottawa,  1910,  p.  18.     Cirkel  (p.  282),  under  laboratory  uses 
does  not  mention  Gooch  crucibles. 


50  APPARATUS  AND  REAGENTS 

to  free  it  from  chlorine,  and  kept,  mixed  with  enough  water  to  form 
a  thin  cream,  in  a  wide-mouth,  glass-stoppered  bottle.  There 
should  be  no  coarse  bits  of  fiber  in  the  final  product. 

Barium  Chloride  (4  ounces). — A  solution  of  20  grams  in  200  c.c. 
of  water  is  used. 

Boric  Acid  (4  ounces). — This  may  be  used  in  the  titration  for 
ferrous  oxide  (p.  186).  It  is  kept  in  the  solid  form. 

Calcium  Carbonate  (1  pound). — This  is  in  powder  form,  not 
too  fine  and  light.  Only  the  very  best  quality  should  be  used, 
and  the  amount  of  alkali  (reckoned  as  NaCl)  in  10  grams  properly 
sampled  should  be  determined  by  the  Smith  method  (p.  193), 
so  as  to  be  able  to  apply  the  proper  correction.  This  must  never 
be  neglected  when  dealing  with  a  new  lot.  If  the  carbonate  is  of 
really  good  quality,  containing  less  than  half  a  milligram  of  NaCl 
in  4  grams,  the  correction  need  not  be  applied,  except  in  very 
accurate  work.  The  amount  of  alkali  in  poor  material  can  be 
much  reduced  by  long  washing  with  hot  water. 

Calcium  Chloride  (1  pound). — This  should  be  in  granular  form, 
for  use  in  drying  apparatus,  and  the  ordinary  "  pure  "  quality  will 
serve. 

Chloroplatinic  Acid  (20  grams). — As  it  is  usually  obtained  this 
contains  about  37  per  cent  of  platinum.  It  is  used  as  a  solution 
containing  either  0.05  or  0.10  gram  of  platinum  per  cubic  centi- 
meter. My  preference  is  for  the  former.  To  make  this,  10  grams 
are  dissolved  in  50  c.c.  of  water  in  the  cold,  filtered  through  a  very 
small  filter,  the  filter  washed  two  or  three  times,  and  the  filtrate 
made  up  to  75  cc.  in  the  reagent  bottle.  The  bottle  and  cap 
should  be  wiped  clean  frequently.  For  the  calculation  of  the 
proper  amount  to  use  in  the  analysis  see  p.  203. 

Chromium  Standard  Solution. — This  is  prepared  by  dissolving 
0.1276  gram  of  pure,  normal  potassium  chromate  (K^CrO-i)  in 
water  and  making  up  to  500  c.c. 

Dimethylglyoxime  (1  ounce). — A  little  of  this  is  dissolved  in 
alcohol  as  needed  for  the  determination  of  nickel. 

Ether,  Ethyl  (8  ounces). — This  should  be  anhydrous. 

Hydrogen  Peroxide  (8  ounces). — Any  good  brand,  such  as 
"  dioxogen,"  containing  at  least  3  per  cent  of  H202,  will  serve.  It 
is  scarcely  necessary  now  to  test  for  fluorine,  which  can  be  done  by 
the  method  suggested  by  Hillebrand  (Bull.  422,  p.  40).  This  is 


REAGENTS  51 

the  only  likely  impurity  that  will  affect  its  use  in  the  analysis. 
As  the  reagent  decomposes  on  standing,  especially  if  in  a  warm 
place  or  in  warm  weather,  the  bottle  should  not  be  too  closely 
stoppered,  and  it  is  best  to  purchase  it  fresh,  in  small  quantities 
from  time  to  time. 

Iron  Sulphide  (2  pounds). — The  ordinary  fused,  granular  or 
stick  form,  is  used  for  generating  hydrogen  sulphide. 

Litmus  Paper. — A  little  of  both  the  blue  and  red  will  be  useful. 
It  may  be  obtained  in  small  book  form  or  in  strips  in  vials,  and  is 
best  kept  in  a  wide-mouth,  glass-stoppered  bottle,  painted  black 
or  kept  in  the  dark. 

Macerated  Paper. — This  is  used  in  the  final  precipitation  with 
ammonia  water  (p.  154),  and  some  is  best  made  up  beforehand, 
as  follows:  Two  or  three  11  cm.  ashless  filter  papers  are  torn  in 
pieces,  placed  in  a  small  beaker,  and  moistened  with  just  enough 
concentrated  hydrochloric  acid  to  wet  them.  After  standing  for 
two  minutes  (not  more),  distilled  water  is  gradually  added,  the 
mass  is  vigorously  stirred,  whereupon  the  paper  disintegrates 
rapidly.  The  product  is  freed  from  the  acid  by  washing  under 
suction  and  kept,  mixed  with  enough  water  to  make  a  cream,  in  a 
small,  wide-mouth  bottle.  It  should  be  properly  labelled  so  as  to 
distinguish  it  from  asbestos. 

Magnesia  Mixture. — This  may  be  made  by  dissolving  10  grams 
of  magnesium  chloride  and  30  grams  of  ammonium  chloride  in 
130  c.c.  of  water  and  adding  70  c.c.  of  ammonia  water;  or  by  dis- 
solving 20  grams  of  magnesium  sulphate  and  40  grams  of  ammo- 
nium chloride  in  160  c.c.  of  water  and  adding  80  c.c.  of  ammonia 
water.  In  either  case  the  solution  is  allowed  to  stand  for  some  days 
and  is  then  filtered. 

Manganese  Standard  Solution. — This  should  contain  2  milli- 
grams of  MnO  in  10  c.c.  It  is  prepared  by  dissolving  0.2228  gram 
of  the  purest,  dry,  potassium  permanganate  in  250  c.c.  of  water. 
After  standing  for  a  day  or  two,  10  c.c.  of  sulphuric  acid  are  added, 
and  the  permanganate  is  reduced  by  the  very  cautious  addition  of 
a  solution  of  sulphur  dioxide  in  water,  until  the  solution  just 
becomes  colorless.  When  cool,  it  is  to  be  diluted  to  exactly  500  c.c. 
in  a  measuring  flask. 

Marble  (5  pounds). — Any  good  grade  of  white  marble,  not 
dolomitic,  in  lumps,  will  serve  to  generate  carbon  dioxide.  As 


52  APPARATUS  AND  REAGENTS 

some  marble  contains  sulphides,  the  gas  must  be  washed  with  a 
solution  of  copper  sulphate,  as  well  as  with  water. 

Methyl  Orange  (1  gram.) — The  solution  is  made  by  dissolving 
20  mgr.  of  the  dye  in  100  c.c.  of  water.  It  is  most  conveniently 
kept  in  a  small  dropping  bottle. 

Perchloric  Acid  (4  ounces). — This  reagent  may  be  needed 
for  the  separation  of  potash  from  soda  (p.  207).  It  may  be  pur- 
chased, but  is  best  prepared  by  Kreider's  method,  which  is  de- 
scribed by  Tread  well.1  This  consists  in  fusing  sodium  chlorate 
until  it  is  changed  into  sodium  perchlorate  and  chloride.  The  melt 
is  dissolved  in  water,  hydrochloric  acid  is  added,  and  the  mixture  is 
evaporated  to  dryness.  The  dry  mass  is  treated  with  an  excess  of 
concentrated  hydrochloric  acid,  the  solution  of  perchloric  and 
hydrochloric  acids  is  filtered  off  from  the  sodium  chloride,  and  the 
filtrate  evaporated  down  until  the  hydrochloric  acid  is  driven  off 
and  white  fumes  of  perchloric  acid  are  evolved.  Purification  from 
potash  is  sometimes  needed.  For  details  the  student  should  con- 
sult Treadwell.  Alcoholic  solutions  of  perchloric  acid  must  not  be 
evaporated  over  a  naked  flame,  for  fear  of  dangerous  explosions; 
evaporation  on  the  steam  bath  is  apparently  safe,  and  evapora- 
tion of  aqueous  solutions  is  not  dangerous. 

Potassium  Nitrate  (1  ounce). — This  is  occasionally  needed  for 
oxidizing  sulphides.  It  is  used  in  a  solid  form. 

Potassium  Permanganate  (1  ounce). — A  solution  of  a  con- 
centration appropriate  for  use  in  rock  analysis  is  made  by  dissolv- 
ing 1  gram  (best  weighed  to  1  mm.)  of  the  pure,  dry  salt  in  1  liter 
of  water.2  The  solution  should  stand  for  one  week,  to  completely 
oxidize  any  organic  matter  in  the  water,  and  is  filtered  through 
previously  ignited  asbestos  (placed  as  a  loose  plug  in  a  rather 
large  funnel)  into  the  stock  bottle,  before  standardization.  One 
c.c.  of  this  solution  will  correspond  to  about  0.0025  Fe20s  or 
0.00225  FeO. 

The  standardization  may  be  effected  with  any  of  the  well- 
known  reagents,  such  as  metallic  iron,  sodium  thio-sulphate, 
oxalic  acid,  and  ammonium,  potassium,  or  sodium  oxalate.  The 

1  Treadwell,  p.  51. 

2  For  descriptions  of  the  preparation  and  different  methods  of  standardiza- 
tion of  the  solution,  see  Treadwell,  pp.  90,  597;  Mellor,  p.  ^193;    Morse, 
p.  459. 


REAGENTS  53 

last  is  the  one  recommended.1  Sodium  oxalate  (Na2C204)  is 
readily  obtainable  pure  and  dry.2  It  is  best  to  dry  it  for  two  hours 
at  130°  in  the  oven.  My  procedure,  which  is  essentially  that  of 
McBride  and  Blum,  except  as  to  temperature,  is  as  follows: 
Three  portions  of  the  sodium  oxalate,  of  about  0.066  gram  each, 
are  weighed  out  to  a  tenth  of  a  milligram,  dissolved  in  water  and 
placed  in  three  400  c.c.  beakers.  To  each  is  added  5  c.c.  of  1  :  1 
sulphuric  acid,  and  the  volume  is  brought  to  about  250  c.c.  They 
are  then  titrated  successively  (from  a  weight  burette),  at  room 
temperature3  with  the  permanganate  solution,  the  liquid  in  the 
beaker  being  stirred  vigorously  and  continuously  during  the 
titration.  The  decolorization  takes  place  very  slowly  at  first, 
more  slowly  than  with  ferrous  oxide,  and  one  must  wait  for  this 
before  the  addition  of  another  portion  of  permanganate.  The 
permanganate  is  to  be  added  in  portions,  at  first,  of  not  more  than 
1  c.c.,  and  is  "  not  to  be  added  more  rapidly  than  10-15  c.c.  per 
minute,  and  the  last  J-l  c.c.  must  be  added  dropwise  with  par- 
ticular care  to  allow  each  drop  to  be  fully  decolorized  before  the 
next  is  introduced. "  The  end-point  is  reached  when  the  first 
pink  blush  is  obtained  that  does  not  disappear  with  a  minute's 
vigorous  stirring.  For  all  but  the  most  accurate  work,  the  esti- 
mation of  "  the  excess  of  permanganate  used  to  cause  an  end-point 
color  by  matching  the  color  in  another  beaker  containing  the  same 
bulk  of  acid  and  water,"  is  an  unnecessary  refinement.  The 
depth  of  color  decided  on  as  the  end-point  may  vary  slightly  with 
different  individuals,  but  that  used  in  the  standardization  must  be 
adhered  to  in  the  iron  determinations. 

The  mean  of  the  three  determinations,  which  should  not  vary 
more  than  0.1  gram,  is  taken,  and  will  be,  for  the  amounts  taken 
above,  about  30  grams  of  permanganate  solution.  As  equal 

1  Cf.  Mellor,  p.  193;  R.  S.  McBride,  Jour.  Am.  Chem.  Soc.,  34,  pp.  394, 
415,  1912,  Bull.  Bur.  Stand.,  81,  p.  611,  1912;   W.  Blum,  Bull.  Bur.  Stand., 
8,  pp.  719,  726,  1912;  Bur.  Stand.  Circ.  No.  40,  1912. 

2  A  thoroughly  reliable,  certified,  standard   sodium   oxalate  is  issued  by 
the  Bureau  of  Standards,  Washington,  D.  C.,  in  bottles  of  120  and  200  grams. 
It  should  be  kept  in  the  original  bottle,  closely  stoppered. 

3  McBride  and  Blum  titrate  at  from  60°  to  80°.     My  preference  is  for  room 
temperature,  because  this  corresponds  to  that  at  which  the  ferrous  oxide  is 
determined,  and  because  of  the  condensation  of  steam  on  the  weighing  burette 
over  hot  water  and  the  possibility  of  not  wiping  it  perfectly  dry  before  weighing. 


54  APPARATUS  AND  REAGENTS 

amounts  of  permanganate  are  required  to  oxidize  1  molecule  of 
Na2C204  (molecular  weight  =  134)  and  2  molecules  of  FeO  (molec- 
ular weight  =144),  the  weight  of  oxalate  per  cubic  centimeter  is. 
to  be  multiplied  by  JfJ=  1.0746,  to  give  the  equivalent  per 
cubic  centimeter  in  terms  of  FeO.  This  divided  by  0.9,  or  mul- 
tiplied by  1.1111,  will  give  the  value  per  cubic  centimeter  for 
Fe2O3. 

The  solution  of  permanganate  is  quite  stable  l  if  kept  out  of 
contact  with  dust  and  reducing  gases,  and  especially  in  the  dark. 
It  is  best  to  paint  the  glass-stoppered  stock  bottle  with  black 
paint,  cover  the  top  with  a  glass  cap,  and  restandardize  it  every 
six  months.  According  to  Blum  the  addition  of  1  per  cent  of 
KOH  increases  its  stability.  The  sodium  oxalate  solution  does 
not  keep,  and  must  be  made  up  fresh  for  each  standardization. 

Potassium  Pyrosulphate  (1  pound). — This  must  be  free  from 
silica,  alumina,  and  iron.  Only  the  fused  salt,  that  is,  the  acid 
potassium  sulphate  (KHS04)  converted  by  heat  into  pyrosul- 
phate  (K2S2O7),  should  be  used.  It  should  not  froth  or  bubble  on 
melting,  due  to  escape  of  water.  If  not  procurable  it  may  be  made 
by  fusing  the  acid  salt,  moistened  with  a  little  sulphuric  acid,  in  a 
platinum  basin  until  there  is  no  frothing  and  white  fumes  of 
sulphur  trioxide  are  given  off.  The  cold  mass  is  broken  up,  on  a 
stone  slab  or  in  a  porcelain  mortar,  but  not  on  an  iron  plate,  into 
rather  small  lumps  and  coarse  powder.  The  sodium  salt  may  also 
be  used,  but  my  preference  is  in  favor  of  the  potassium  salt. 

Potassium  Thiocyanate  (1  ounce). — A  solution  made  by 
dissolving  10  grams  in  100  c.c.  of  water  is  used  for  detecting 
ferric  salts. 

Potassium  Titanofluoride  (1  ounce). — This  is  preferable  to 
titanium  dioxide  for  the  preparation  of  the  standard  titanium 
solution,  as  it  is  more  readily  obtained  pure.  It  should  be  heated 
at  150°  for  two  hours  to  render  it  anhydrous,  and  kept  in  a  well- 
stoppered  bottle. 

Silver  Nitrate  (1  ounce). — A  solution  containing  3  grams  to 
the  liter  is  used  in  the  colorimetric  determination  of  manganese. 
Five  hundred  c.c.  of  this  may  be  made  up.  About  100  c.c.  of  a 

1  Cf.  Mellor,  p.  196;  Treadwell,  p.  603.  A  standard  solution  of  mine  which 
gave  a  value  of  0.002497  gram  Fe2O3  per  gram  on  January  12th,  gave  one  of 
0.002480  on  November  1st  of  the  same  year. 


REAGENTS  55 

stronger  solution  may  be  kept  in  a  small  dropping  bottle  for  testing 
filtrates  for  chlorine. 

Soda  Lime  (1  pound). — This  should  be  of  good  quality  and 
granulated. 

Sodium  Acetate  (4  ounces) . — This  is  kept  in  the  solid  form. 

Sodium  Ammonium  Phosphate  (microcosmic  salt).  (8  ounces). 
— This  is  kept  in  the  solid  form  and  the  solution  is  made  as  needed. 

Sodium  Carbonate  (2  pounds). — Only  the  dry,  anhydrous  salt, 
of  the  best  obtainable  quality,  is  to  be  used.  A  new  lot  should 
always  be  tested,1  especially  for  silica,  alumina,  and  iron.  I  have 
now  abandoned  the  mixture  of  sodium  and  potassium  carbonates, 
formerly  recommended  and  use  the  sodium  carbonate  alone. 

Sodium  Oxalate  (4  ounces). — It  is  important  to  have  this  per- 
fectly pure  and  dry.  That  furnished  by  the  Bureau  of  Standards 
is  the  most  reliable  and  should  be  obtained  if  possible.  A  bottle 
containing  120  grams  will  suffice. 

Sulphur  Dioxide. — If  this  is  used  for  the  reduction  of  ferric 
to  ferrous  oxide  it  is  better  to  use  the  gas,  as  the  solution  does  not 
keep  well  on  long  standing.  The  gas  may  be  obtained  in  7-pound 
cylinders,  or  small  quantities  of  fresh  solution  may  be  obtained 
from  time  to  time. 

Titanium  Standard  Solution. — This  should  contain  1  centi- 
gram of  TiO2  in  10  c.c.  It  is  best  made  from  potassium  titano- 
fluoride  (K^TiFo),  which  is  procurable  in  a  satisfactory  degree  of 
purity.  The  salt  should  be  recrystallized  and  heated  for  an  hour 
•or  two  at  150°  to  render  it  anhydrous.  As  nearly  as  possible  1.5 
grams  2  of  the  anhydrous  salt  (which  contains  titanium  equivalent 
to  just  33.33  per  cent  of  TiO2)  is  weighed  out  into  a  platinum  dish 
or  large  crucible  and  evaporated  four  or  five  times,  with  5-gram 
portions  of  sulphuric  acid  (1  :  1),  until  fumes  of  sulphur  trioxide  are 
given  off  and  almost,  but  not  quite  to  dryness.  All  the  HF  must 
be  expelled.  When  cool  the  residue  in  the  crucible  is  mixed  first 
with  5  c.c.  of  1  :  1  sulphuric  acid  then,  cautiously  with  water, 
enough  sulphuric  acid  added  (if  necessary)  to  make  up  at  least 
5  per  cent  of  final  volume,  and  when  cool  the  whole  is  diluted  to 
500  c.c.  in  a  measuring  flask.  Two  50  c.c.  portions  are  diluted 

1  Hillebrand,  Bull.  422,  p.  40. 

2  Mellor  (p.  205,  note  4)  has  made  a  slip  in  the  figures  given  in  his  direc- 
tions for  preparing  the  solution. 


56  APPARATUS  AND  REAGENTS 

with  a  little  water  and  precipitated  at  a  boiling  temperature  with 
ammonia  water,  the  precipitate  is  well  washed,  ignited  and  weighed 
as  TiO2.  As  iron  is  a  frequent  impurity,  it  is  best  to  bring  the  two 
portions  of  TiCb  into  solution  (p.  159)  separately  by  fusion  with 
pyrosulphate  and  determine  the  amount  of  Fe2Os  present.  This, 
of  course,  is  to  be  deducted  from  the  apparent  weight  of  TiO2. 
If  the  standard  proves  not  to  contain  exactly  0.01  gram  of  Ti(>2  in 
10  c.c.,  it  is  better  to  use  its  actual  value  in  the  calculations  rather 
than  to  dilute  it  to  the  proper  strength. 

The  purest  titanium  dioxide  may  also  be  used,  0.5  gram  being 
brought  into  solution,  either  by  long  fusion  with  pyrosulphate  or, 
better,  by  evaporation  with  a  mixture  of  hydrofluoric  and  sul- 
phuric (1:1)  acids  and  subsequent  repeated  evaporations  with 
sulphuric  acid  as  above.  The  solution  so  prepared  should  always 
be  checked  for  iron,  as  the  dioxide  is  more  likely  to  be  contaminated 
with  this  impurity  than  the  titanofluoride. 

The  standard  solution  of  titanium  keeps  well  if  strongly  acid 
with  sulphuric  acid  (at  least  5  per  cent),  but  it  is  well  to  determine 
its  titanium  content  from  time  to  time. 

Water. — Only  pure,  distilled  water  is  to  be  used  throughout 
the  analysis,  and  it  will  be  understood  that  this  is  referred  to 
wherever  this  substance  is  mentioned  in  this  book.  The  use  of 
impure  water  will  vitiate  the  results  of  any  analysis,  and  too  great 
precautions  cannot  be  taken  to  provide  for  an  ample  supply  of 
pure,  distilled  water.  This  important  point  is  neglected  only  too 
frequently.  The  water,  after  distillation,  should  be  stored  in 
large  bottles  of  some  resistance  glass,  and  should  be  used  as  fresh 
as  may  be  practicable. 

Zinc  Oxide  (1  ounce). — A  little  of  this  may  be  dissolved  in 
ammonia  water  as  needed  in  the  rarely  executed  determination  of 
fluorine. 


PART    III 

THE   SAMPLE 

1.  SELECTION  IN  THE  FIELD 

SINCE  the  object  of  the  chemical  analysis  of  rocks  is  to  ascer- 
tain the  chemical  composition  of  a  body  of  rock,  it  is  of  funda- 
mental importance  that  the  specimen  selected  for  analysis,  and 
the  material  analyzed,  be  truly  representative  of  the  mass  under 
investigation. 

If,  for  instance,  an  igneous  mass  is  not  uniform  in  character, 
and  the  specimen  is  selected  from  some  extreme  phase  of  variation, 
it  is  obvious  that  an  analysis  of  this  will  not  give  a  just  idea  of 
the  character  of  the  mass  as  a  whole.  Again,  in  analyzing  a 
diorite,  for  instance,  the  specimen  may  be  so  small  or  selected 
with  so  little  care  that  it  contains  a  larger  proportion  of  horn- 
blende, let  us  say,  than  the  average  of  the  mass;  or  the  specimen 
of  a  quartz-porphyry  may  carry  only  a  few  of  the  abundant, 
but  readily  broken  out,  quartz  phenocrysts  and  a  disproportionate 
amount  of  ground-mass.  It  is  evident  that  an  analysis  made  on 
such  inadequate  material,  however  skilfully  it  may  be  executed, 
cannot  represent  the  true  composition  of  the  rock-mass.  It  is 
seen,  therefore,  that  the  proper  selection  of  the  material  for 
analysis  depends  on  two  factors :  the  selection  of  the  representative 
specimen  in  the  field,  the  amount  of  material  taken  and  the  proper 
sampling  of  this  for  use  in  making  the  analysis. 

While  the  selection  in  the  field  is  quite  apart  from  the  laboratory 
processes,  yet  its  importance  is  so  great  as  a  preliminary  to  the 
analysis  that  it  demands  some  discussion.  This  is  the  more  called 
for  since  the  petrologist  will  usually  collect  his  own  material,  for 
analysis  either  by  himself  or  by  others,  and,  as  has  been  said  else- 
where, "  the  evidence  is  conclusive  that  the  specimen  analyzed 
has  often  been  collected  with  no  reference  to  this  point,  this  fact 
greatly  diminishing  the  value  of  the  analytical  work  afterward 

57 


58  THE  SAMPLE 

expended  on  it."  In  selecting  a  representative  specimen  in  the 
field  attention  must  be  paid  to  two  points:  the  uniformity  of  the 
mass,  especially  in  regard  to  mineral  composition  as  well  as  to 
texture,  and  the  freshness  of  the  rock. 

Uniformity  of  the  Rock-mass. — If,  as  is  true  in  the  majority  of 
cases,  the  igneous  mass  is  sensibly  uniform  throughout  its  extent, 
specimens  should  be  taken  from  several  parts,  when  possible,  in 
order  to  test  the  uniformity  with  the  microscope.  For  an  analysis 
representing  the  composition  of  such  a  uniform  body  of  igneous 
rock,  either  portions  of  several  specimens  from  different  parts 
may  be  mixed,  or  the  analysis  may  be  made  on  a  single  specimen, 
which  is  considered  to  be  representative  of  the  whole  in  the  judg- 
ment of  the  petrographer,  both  as  decided  on  in  the  field  and  as 
confirmed  by  the  microscope. 

As  to  the  former  procedure  it  may  be  said  that  no  decisive 
check  of  one's  results  will  be  possible  in  the  future,  and  that  it  is 
by  no  means  certain  that  a  mixture  of  several  specimens  really 
represents  the  composition  of  the  whole  better  than  does  a  single 
specimen  which  has  been  carefully  selected  with  this  object  in 
view. 

Furthermore,  the  analysis  of  a  single  specimen  can  be  corre- 
lated with  its  mode,  or  quantitative  mineral  composition,  as  deter- 
mined by  the  microscope  in  thin  section,  and  thus  both  will  be 
available  for  use  in  physico-chemical  discussion.  To  put  it  briefly, 
the  analysis  should  represent  the  chemical  composition  of  a  speci- 
men and  the  specimen  the  composition  of  the  rock-mass,  so  that 
petrology  and  petrography,  both  in  their  broader  and  narrower 
aspects,  may  avail  themselves  of  the  data.1 

In  all,  or  nearly  all,  cases,  therefore,  and  wherever  possible,  a 
single  specimen  should  be  selected  for  analysis  after  due  compari- 
son with  others  from  the  same  mass  and  consideration  of  its  repre- 
sentative character.  The  specimen  should  be  taken,  if  possible, 
from  a  mass  of  rock  in  place,  and  not  from  loose  boulders  or  talus 
slopes,  unless  these  are  the  only  sources  available  and  it  is  defi- 
nitely known  that  they  do  come  from  the  mass  under  investigation. 

If  the  mass  is  not  uniform,  but  is  composed  of  portions  with 

1  The  question  of  the  variability  of  an  apparently  uniform  rock-mass,  and 
the  representativeness  of  a  single  specimen  is  briefly  discussed  in  Washington, 
Prof.  Paper,  99,  p.  11. 


SELECTION  IN   THE  FIELD  59 

different  characters,  such  as  a  composite  dike  or  a  stock  with 
marginal  facies,  representative  specimens  of  the  different  facies 
should  be  collected  and  an  analysis  made  of  each,  whether  the 
differences  be  apparently  only  textural  or  those  due  to  mineral 
composition.  If  in  any  way  feasible,  as  close  an  estimate  as  the 
conditions  allow  should  be  made  of  the  relative  thickness,  areas  or 
volumes  of  each  facies.  While  the  possibility  of  doing  this  depends 
to  a  large  extent  on  the  conditions  of  exposure  due  to  the  chances 
of  erosion  and  denudation,  yet  it  is  of  such  importance  in  the  inves- 
tigation of  certain  theoretical  questions  of  petrology  that  special 
endeavor  should  be  made  to  arrive  at  the  facts. 

In  any  case,  whether  the  mass  be  uniform  or  composed  of 
several  facies,  the  specimen  should  be  taken,  if  possible,  from  some 
definite  locality,  one  which  can  be  described  or  named  so  that  it 
can  be  readily  identified  by  others,  and  also  one  whose  accessibility 
is  not  likely  to  be  lost  through  building  or  other  operations. 
Quarries  naturally  are  especially  favorable  spots,  as  fresh  speci- 
mens are  easily  obtained,  and  they  are  of  such  a  permanent  nature 
as  to  be  readily  identified,  in  most  cases,  by  future  investigators. 
In  rapid  reconnaissance  or  exploration  one  has,  of  course,  mostly 
to  be  content  with,  and  often  thankful  for,  chance  specimens. 

Freshness  of  the  Rock. — The  action  of  atmospheric  agencies 
on  rocks  may  vary  from  the  changes  to  which  Merrill l  attaches 
the  specific  term  "  alteration,"  in  which  "  the  rock-mass  as  a 
whole  retains  its  individuality,"  but  is  changed  mineralogically, 
with  the  production  of  secondary  minerals,  chlorite,  sericite, 
zeolites,  serpentine,  limonite,  etc.,  to  those  embraced  under  what 
Merrill  calls  "  weathering,"  "  involving  the  destruction  of  the 
rock-mass,"  and  its  ultimate  resolution  into  sands  and  clays. 
The  mass  resulting  from  such  changes,  either  of  alteration  or 
weathering,  can  be  analyzed  by  the  same  methods  and  with 
equal  facility  as  can  a  perfectly  fresh  rock,  but  it  is  evident  that 
the  results  will  not  represent  the  composition  of  the  original 
magma  or  unaltered  rock  body. 

While  it  is  true  in  general  that  only  specimens  of  fresh  (unal- 
tered or  un weathered),  rock  should  be  chosen  for  analysis,  unless 
the  study  of  such  secondary  changes  is  the  object  in  view,  yet  it  is 
at  times  somewhat  difficult  to  decide  whether  a  rock  is  fresh  enough 
1  G.  P.  Merrill,  Rocks,  Rock-weathering  and  Soils,  p.  174,  1897. 


60  THE  SAMPLE 

for  analysis  or  not.  In  general  it  may  be  said  that,  for  the  study 
of  igneous  rocks,  all  weathered  specimens  are  to  be  rejected,  that 
is  to  say,  those  in  which  the  rock-mass  has  been  formally  broken 
down.  In  the  case  of  alteration,  specimens  should  be  rejected 
where  the  original  color  is  decidedly  changed,  as  where  the  rock  is 
of  a  rusty  brown  through  the  abundant  production  of  limonite,  or 
green  through  that  of  chlorite.  Specimens  which  effervesce  with 
hydrochloric  acid,  either  cold  or  on  warming,  or  whose  vesicles 
contain  calcite  or  zeolites,  are  likewise  to  be  shunned. 

Specimens  should,  therefore,  be  taken  (if  possible)  from  the 
interior  of  the  mass  and  not  from  the  surface,  and  portions  of 
specimens  from  near  the  surface,  where  the  rock  has  been  exposed 
to  atmospheric  agencies,  are  to  be  rejected. 

In  rocks  which  appear  megascopically  to  be  quite  fresh,  the 
microscope  may  reveal  the  presence  of  secondary  minerals,  the 
products  of  alteration,  as  sericite,  chlorite,  serpentine  or  limonite. 
Although  considerable  latitude  must  be  left  to  the  judgment  of 
the  petrographer  in  deciding  this  matter,  yet  if  such  minerals  are 
present  to  any  considerable  extent,  the  rock  must  be  regarded  as 
unfit  for  chemical  analysis,  unless  fresh  material  is  unattainable. 
This  last  state  of  affairs  is  especially  apt  to  be  true  of  the  most 
basic  rocks,  such  as  alnoites,  picrites,  peridotites,  and  pyroxenites, 
which  contain  a  large  amount  of  the  easily  oxidizable  ferrous  iron, 
and  of  which  few  perfectly  fresh  occurrences  are  known  or  have 
been  analyzed.  For  lack  of  better  material,  one  must  often 
analyze  specimens  of  such  rocks  that  are  far  from  fresh,  but  the 
results,  while  not  to  be  regarded  as  wholly  satisfactory,  may  yet 
be  of  some  service. 

The  results  of  alteration  are  usually  most  clearly  shown  in  the 
analysis  by  the  figures  for  H^O  or  C02,  or  both.  Where  these  are 
high  the  material  analyzed  must  be  considered  as  having  been 
more  or  less  altered,  whether  this  appears  in  the  description  or 
not,  with  the  exception  of  certain  cases  mentioned  below.  While 
it  is  impossible  to  state  in  exact  figures  the  limits  of  allowable 
alteration,  until  the  subject  is  further  studied,  it  may  be  held 
provisionally  that  H2O  can  go  up  to  2  or  3  per  cent  and  CO2  to 
0.5  per  cent,  without  the  rock  being  considered  to  be  so  altered  as 
to  seriously  affect  the  value  of  the  analysis.  It  must  also  be  borne 
in  mind  that  a  rock  can  be  more  or  less  profoundly  altered,  and  yet 


SELECTION  IN  THE  FIELD  61 

show  comparatively  low  figures  for  these  two  constituents,  though 
this  is  not  often  to  be  expected. 

The  exceptional  cases  just  referred  to  consist  of  rocks  com- 
posed in  part  of  primary  minerals  which  contain  either  hydroxyl 
(as  muscovite  and  biotite),  water  of  crystallization  (analcite), 
or  carbon  dioxide  (cancrinite) .  With  rocks  carrying  analcite, 
which  is  the  only  zeolite  that  apparently  can  exist  as  a  primary 
mineral,  the  EbO  may  amount  to  3  or  4  per  cent,  and  yet  the 
mass  be  to  all  appearances  perfectly  fresh,  and  in  some  almost 
certainly  so,  as  Pirsson  has  shown  of  the  monchiquites.  Highly 
vitreous  lavas,  as  perlites  and  tachylites,  may  contain  several 
per  cent  of  water  and  yet  be  perfectly  fresh;  while  water  often 
occurs  as  inclusions,  as  in  the  quartzes  of  many  granites.  Can- 
crinite-bearing  rocks  may  have  more  than  1  per  cent  of  C02  and 
yet  be  quite  unaltered,  as  far  as  one  may  judge  from  the  micro- 
scope, so  that  it  is  entirely  possible,  if  not  probable,  that  this 
mineral  is  a  primary  constituent  in  some  cases.  The  existence  of 
calcite  as  an  undoubtedly  primary  mineral  has  not  been  estab- 
lished as  yet,  though  recently,  rocks  have  been  described  in  which 
its  occurrence  as  such  seems  to  be  probable. 

In  discussing  the  subject  of  analyses  of  altered  rocks  we 
may  advert  to  a  phase  of  the  matter  which  is  of  some  importance. 
When  a  rock  is  not  fresh  it  is  sometimes  assumed  that  the  original 
composition  can  be  arrived  at  by  deducting  the  amounts  of  H^O 
and  C02  and  calculating  the  remainder  to  100  per  cent.  This 
assumption  is  generally  quite  unwarranted,  since  the  processes  of 
alteration  are  usually  by  no  means  simply  the  result  of  the 
addition  of  the  two  substances  mentioned.  On  the  contrary, 
they  are  complex  and  produce  changes  of  greater  or  less  magnitude 
in  the  proportions  of  some  or  all  of  the  other  constituents. 
These  may  be  additive,  as  when  calcite  is  deposited  in  rocks  by 
means  of  percolating  waters  carrying  calcium  bicarbonate  in  solu- 
tion, or  they  may  be  subtractive,  as  when  kaolinization  of  a  feld- 
spar takes  place  with  resultant  loss  of  alkalies  or  lime.  In  any 
case  it  is  almost  universally  true  that  the  processes  of  rock  degen- 
eration affect  all  or  nearly  all  of  the  chemical  constituents,1  and 
that  the  assumption  that  such  is  not  true  is  quite  unwarranted  by 
the  known  facts. 

1  Cf.  Merrill,  Rocks,  Rock-weathering,  and  Soils,  pp.  234  to  240. 


62  THE  SAMPLE 


2.  AMOUNT  OP  MATERIAL 

As  has  been  said  above,  the  representative  character  of  the 
specimen  depends,  after  proper  selection  in  the  field  supple- 
mented by  the  use  of  the  microscope,  upon  the  amount  of  material 
which  is  taken  for  pulverization  in  preparation  for  the  analysis. 
The  weight  of  the  sample  which  will  adequately  represent  the 
average  of  the  rock-mass  varies  with  the  texture  of  the  rock,  and 
especially  with  its  granularity,  that  is,  the  size  of  its  component 
mineral  particles. 

It  may  first  be  noted  that  10  grams  of  rock  powder  should  be 
available  for  the  purpose  of  analysis,  and  this  amount  may  be 
increased  to  20  or  30  grams  if  the  analysis  is  to  be  very  complete, 
since  the  determination  of  some  of  the  rarer  constituents  demands 
the  use  of  two  or  more  grams  of  powder.  Indeed,  it  is  always  a 
wise  precaution  to  have  20  grams  on  hand,  in  vi^w  of  the  possible 
necessity  for  the  duplicate  determination  of  some  of  the  constit- 
uents, or  even  the  making  of  a  second  complete  analysis. 

It  is  often  impossible  to  obtain  anything  like  this  amount  of 
material,  for  the  analysis  of  minerals  or  meteorites  and  the  analyst 
must  be  content  with  smaller  quantities,  sometimes  much  less  than 
a  gram  for  the  whole  analysis.  With  rocks,  on  the  other  hand, 
there  is  usually  an  ample  supply,  so  that  the  analyst  has  generally 
no  reason  for  stinting  himself.  In  this  way  a  number  of  constit- 
uents can  be  easily  determined  in  separate  portions,  which  could 
only  be  accomplished  by  the  use  of  longer  and  more  complex 
methods  if  it  were  necessary  to  determine  them  in  a  single  portion. 

The  texture  of  rocks  varies  within  such  wide  limits  that  it 
is  impossible  to  give  exact  figures  as  to  the  amount  of  material  that 
is  representative.  Much  must  be  left  to  the  judgment  of  the 
petrographer.  Speaking  generally,  and  almost  without  exception, 
the  finer  grained  and  less  porphyritic  the  rock,  the  smaller  will 
be  the  amount  of  material  necessary  to  be  representative. 

Twenty  or  thirty  grams  of  chips  or  fragments  will  be  ample 
for  very  fine-grained,  aphanitic  or  glassy  rocks,  as  many  basalts, 
trachytes,  and  obsidians,  especially  if  they  are  not  porphyritic,  or 
very  finely  so.  With  more  coarsely  granular  rocks,  such  as  gran- 
ites, syenites  and  diorites,  a  larger  quantity  must  be  taken,  depend- 
ing on  the  coarseness  of  the  grain.  This  amount  may  vary  from 


AMOUNT  OF  MATERIAL  63 

50  grams  of  a  medium-grained  rock  to  100  or  even  more  if  the 
grain  is  coarse.  In  some  cases,  as  in  pegmatites,  the  grain  may  be 
so  large  that  only  a  whole  hand  specimen,  or  even  several  kilograms, 
will  adequately  represent  the  true  composition.  Very  exception- 
ally the  crystals  may  be  of  such  gigantic  size  that  the  relative 
proportions  of  the  various  minerals  must  be  estimated  from  a  flat 
exposure  and  corresponding  portions  of  the  several  minerals 
weighed  out  and  mixed.  Fortunately  this  last  will  be  rarely 
necessary,  and  results  obtained  thus  could  be  regarded  as  but 
approximations  to  the  truth. 

If  the  rock  is  porphyritic  this  feature  involves  the  taking 
of  a  larger  quantity  than  would  be  necessary  if  the  grain  of  the 
whole  were  that  of  the  ground-mass.  If  the  phenocrysts  are 
very  small,  only  a  few  millimeters  in  diameter,  and  close  together, 
as  in  many  andesites  and  basalts,  20  or  30  grams  will  be 
sufficient.  As  they  get  larger,  and  if  more  widely  scattered, 
more  must  be  taken,  from  50  or  100  grams  to  larger  quantities. 
With  porphyritic  rocks  also,  care  must  be  taken  that  brittle 
or  loosely  attached  phenocrysts,  as  of  feldspar,  quartz  or  leucite  do 
not  fall  out,  so  as  to  yield  a  disproportionate  amount  of  ground 
mass  in  the  material  for  analysis. 

3.  PBEPARATION  OF  THE  SAMPLE 

Sampling. — If  the  rock  is  so  fine-grained  or  so  slightly  porphy- 
ritic that  only  up  to  about  50  grams  of  fragments  is  considered 
to  be  representative,  the  whole  amount,  broken  up  if  necessary, 
may,  without  sampling,  be  crushed  and  ground,  as  described  below. 

If,  however,  the  representative  amount  is  much  more  than  this, 
say  100  grams  or  over,  the  final  crushing  and  grinding  of  the  whole 
will  consume  an  inordinate  amount  of  time,  so  that  sampling  is 
advisable  to  obtain  the  smaller  amount  needed  for  an  analysis 
that  will  be  representative. 

This  may  be  done  by  breaking  up  the  piece  or  pieces  of  rock, 
if  large,  with  a  hammer  over  a  large,  clean  sheet  of  paper,  until  the 
largest  is  small  enough  to  be  put  in  the  steel  mortar.  The  larger 
fragments,  say  down  to  one-half  of  a  centimeter,  in  greatest  dimen- 
sion, are  crushed,  one  at  a  time,  to  smaller  fragments  and  coarse 
powder,  in  the  Ellis  mortar,  and  emptied  out  on  the  sheet  of  paper. 


64  THE  SAMPLE 

The  whole  mass  is  then  well  mixed  with  a  steel  spatula  and  shaped 
into  a  low  cone.  This  is  " quartered"  with  the  spatula,  and  if  the 
quarter  is  more  than  about  50  grams,  the  process  is  repeated 
(more  than  once  if  necessary)  on  this,  until  a  proper  amount  of 
sample  has  been  obtained. 

Methods  of  Pulverization. — For  analysis  the  sample  of  the  rock 
must  be  reduced  to  rather  fine  powder,  in  order  that  it  may  be 
easily  and  completely  decomposed  by  the  reagents  used  and  thus 
brought  into  condition  for  solution.  To  accomplish  this,  one  of 
three  methods  may  be  followed. 

The  first  is  that  formerly  used  in  the  laboratory  of  the  U.  S. 
Geological  Survey.1  The  rock  is  first  crushed  to  small  fragments 
and  powder  by  means  of  a  hardened  steel  hammer  on  a  hardened 
steel  plate.  The  plate  is  10  cm.  square  and  4J  cm.  thick.  The 
rock  fragment  is  surrounded  by  a  steel  ring,  6  cm.  in  internal  diam- 
eter and  2  cm.  high,  intended  to  prevent  the  flying  and  loss  of 
fragments.  After  reduction  in  this  way  to  small  particles  and 
powder,  the  whole  is  ground  down  by  hand  in  an  agate  mortar,  in 
small  portions  at  a  time.  As  will  be  seen  later,  the  great  defect 
in  this  method  is  the  unavoidable  and  probably  very  serious  loss 
of  small  fragments  and  dust,  both  in  the  preliminary  crushing, 
which  the  ring  does  not  wholly  prevent,  and  especially  in  the  final 
grinding. 

The  second  method  is  that  described  by  Hillebrand  as  the  one 
in  use  in  the  Survey  laboratory.2  The  rock,  broken  into  small 
pieces,  is  crushed  in  an  Ellis  mortar  (p.  41),  the  pestle  being  of 
the  knob  type.  In  this  it  is  crushed  to  a  powder  fine  enough  for 
most  of  the  portions  used  for  the  analysis.  When  the  decomposi- 
tion calls  for  a  finer  powder,  small  portions  are  ground  down  by 
hand  in  an  agate  mortar;  the  material  may  be  sifted.  The 
use  of  mechanical  grinders  has  been  abandoned  in  the  Survey 
laboratory. 

In  the  third  method,  which  is  the  one  I  have  followed  for 
many  years,  the  rock,  broken  into  small  pieces,  is  crushed  in  a 
hardened-steel  mortar  much  as  in  the  Survey  method  but  a  light 
hammer  is  used.  After  the  first  crushing  of  the  whole,  it  is  sifted 
through  silk  bolting  cloth,  the  oversize  being  again  crushed  and 

1  Hillebrand,  Bull.  176,  p.  31. 

2  Hillebrand,  Bull.  422,  p.  46. 


PREPARATION  OF  THE  SAMPLE  65 

sifted,  and  this  operation  is  repeated  until  only  a  gram  or  so  of 
oversize  portion  remains,  which  is  ground  by  hand  in  an  agate 
mortar.1 

In  considering  the  relative  merits  of  these  methods,  two  factors 
must  be  discussed,  as  on  them  depends  the  attainment  of  the  object 
in  view,  namely,  the  production  of  a  rock  powder  that  will  repre- 
sent as  accurately  as  possible  the  composition  of  the  rock  specimen 
taken.  These  two  factors  are:  contamination  from  the  mortars, 
and  the  loss  caused  by  the  flying  off  of  fragments  and  dust. 

Contamination  by  iron  derived  from  the  steel  mortar  and  pestle 
or  plate  and  hammer  is  likely  to  be  more  serious  than  that  by  silica 
derived  from  the  agate.  This  is  because,  in  nearly  all  igneous 
rocks,  silica  is  present  in  much  greater  amount  than  are  the  iron 
oxides,  and  the  introduction  of  metallic  iron  would  affect  the  ratio 
of  ferrous  or  ferric  oxide.  Furthermore,  I  believe  that  the  liability 
to  contamination  by  iron  is  more  likely  to  be  relatively  greater 
than  that  by  silica,  in  spite  of  Hempel's  experiments.2  My  first 
hardened  steel  "  diamond  "  mortar  (not  made  by  Ellis),  shows 
greater  signs  of  abrasion,  though  it  is  but  slight,  than  my  small 
agate  mortar,  though  both  have  been  in  use  concomitantly  for 
the  same  period  of  over  twenty  years,  the  latter  indeed  some  years 
longer. 

If  the  steel  is  properly  hardened,  and  the  crushing  done  by 
strictly  vertical  blows,  the  contamination  by  iron  through  abrasion, 
is  slight,  so  slight  indeed  as  to  be  negligible.3  There  is,  however,  a 
very  slight  contamination,  especially  on  long-continued  crushing, 
even  though  this  be  properly  done,  as  Hillebrand  shows.  In  any 
case,  this  source  of  error  would  seem  to  be  unavoidable.  The 
rock  must  be  reduced  to  powder  for  analysis,  and  hardened  steel 
is  at  present  the  only  practicable  material  of  which  to  make  the 
mortar.4 

It  is  essential,  and  most  important  to  remember,  that  there 
must  be  absolutely  no  rubbing  or  rotary  motion  of  the  pestle 

1  Hillebrand  (p.  52)  has  made  a  slight  slip  in  his  description  of  this  method. 
Only  the  last,  small  portion  of  oversize  material  is  ground  down  in  agate,  not 
the  whole  amount  of  powder,  as  he  states. 

2  Cf.  Hillebrand,  Bull.  422,  p.  56. 

3  Cf.  Hillebrand,  Bull.  422,  p.  51. 

4  Osmiridium  and  tungsten  would  seem  to  be  out  of  the  possibility  of  gen- 
eral realization. 


66  THE  SAMPLE 

during  the  crushing.  Any  such  motion  will  inevitably  introduce 
iron  into  the  specimen. 

Contamination  by  silica  from  the  agate  is  very  slight.  It  is 
quite  negligible  if  the  grinding  is  not  long  continued.  Experi- 
ments by  Allen,1  in  which  200  grams  of  quartz  sand  were  ground 
for  19.5  hours  in  an  agate  mortar,  showed  a  loss  by  abrasion  to  the 
pestle  and  mortar  amounting  to  .1455  per  cent  of  the  quartz. 
With  small  amounts  and  with  only  a  few  minutes  grinding  the 
contamination  is  certainly  negligible.2 

In  my  opinion  the  use  of  mechanical  grinders  is  to  be  avoided. 
Apart  from  the  cost,  the  danger  of  contamination  by  oil  or  metal 
is  always  present,  in  spite  of  safeguards.  Furthermore,  the  long- 
continued  grinding  for  which  they  are  used  is  not  only  unnecessary 
for  the  whole  of  the  sample,  but  introduces,  besides  silica,  the  pos- 
sible errors  of  oxidation  of  the  ferrous  oxide  (p.  183)  and  the  ad- 
sorption of  water  by  the  fine  powder.3  Water,  on  the  other  hand, 
may  be  expelled  from  hydrated  minerals,4  and  carbon  dioxide  from 
calcite,5  by  long-continued  grinding. 

If  the  rock  is  sensibly  homogeneous,  like  an  obsidian  or  a  very 
fine-grained  basalt,  the  loss  of  fragments  incidental  to  crushing  or 
to  the  grinding  of  a  coarse  powder  will  not  materially  affect  the 
composition  of  the  final  material.  If,  however,  the  rock  is  visibly 
non-homogeneous,  phaneric  or  porphyritic,  the  change  in  composi- 
tion so  caused  may  be  serious.  The  tough  minerals,  such  as  pyrox- 
ene and  amphibole,  crush  less  readily  but,  when  broken,  they  tend 
to  fly  more  than  the  less  tough  ones,  such  as  quartz,  feldspar, 
nephelite,  or  leucite,  or  glass.  If  not  confined  they  will  be  lost  in 
part,  and  will  also,  in  their  flight,  carry  away  or  drive  out  some  of 
the  surrounding  powder,  which  would  consist  in  greater  part  than 
they,  of  glass,  quartz  or  feldspathic  minerals.6 

1  Hillebrand,  Bull.  422,  p.  56. 

2  The  estimate  of  about  0.3  per  cent  of  silica  introduced,  as  given  by 
Connor  (XII  Cong.  Geol.  Int.,  C.  R.,  p.  885,  1914),  is  surely  higher  than  in  my 
experience  and  may  be  due  to  special  conditions. 

3  Cf.  Day  and  Allen,  Carnegie  Publ.,  No.  31  p.  56,  1905;  Hillebrand,' Bull. 
422,  p.  64.     E.  T.  Allen  has  found  that  optical  glasses,  on  long-continued 
grinding  in  a  mechanical  grinder  may  absorb  as  much  as  1  per  cent  of  water. 

4  Hillebrand,  Bull.  422,  p.  64. 

5  Johnston  and  Niggli,  Jour.  Geol.,  21,  p.  614,  1913. 

6  The  influence  of  hardness  or  brittleness  on  change  in  composition  during 


PREPARATION  OF  THE  SAMPLE  67 

It  is  not  known  by  experiment  in  what  direction  such  losses 
would  tend.  Indeed,  from  this  cause,  the  tendency  to  greater 
loss  in  either  direction  would  probably  be  irregular  and  dependent 
on  the  different  mineral  and  textural  characters  of  the  rocks.  In 
either  case  flying  fragments  would,  however,  tend  to  introduce  an 
error  in  the  composition  of  the  final  material. 

The  loss  from  dust  blown  or  drifting  away  in  the  air  must  also 
be  taken  into  consideration.  This  will,  almost  certainly,  be  more 
largely  made  up  of  salic  than  femic  minerals,  because  of  the 
greater  brittleness  and  lower  specific  gravity  of  the  former. 

In  the  first  method  above,  the  contamination  by  iron  would 
probably  be  notable,  and,  what  is  a  still  more  serious  matter, 
the  loss  by  flying  fragments,  as  well  as  by  dust,  is  obviously  apt 
to  be  so  great  that  the  method  should  be  abandoned,  as  it  has  been. 

The  second  method  involves  prolonged  crushing  in  the  steel 
mortar  to  bring  the  whole  powder  to  a  state  of  fineness  (some- 
thing like  25  meshes  to  a  centimeter)  that  will  permit  ready  attack 
by  sodium  carbonate.  This  renders  the  sample  liable  to  contami- 
nation by  iron  in  direct  ratio  to  the  time  expended  in  crushing.  If 
the  crushing  is  not  carried  out  to  this  extent,  but  if  the  powder 
yielded  by  the  steel  mortar  is  ground  down  in  agate,  loss  and  con- 
sequent change  in  composition  by  flying  fragments  would  be 
inevitable,  as  is  evident  from  the  considerations  above  and  as  can 
be  easily  verified. 

In  the  third  method  the  crushing  in  the  steel  mortar  is  briefer 
than  in  the  second,  because  the  larger  fragments  are  separated  by 
sifting  from  the  powder,  and  thus  in  the  subsequent  re-crushings 
are  not  protected  by  the  cushioning  effect  of  this  and  are  there- 
fore more  quickly  pulverized.1  The  sifting,  if  carried  out  in  a 
quiet  place,  free  from  draughts,  and  over  a  large  sheet  of  paper, 

pulverization  has  been  incidentally  studied  by  S.  Zaleski  (Tsch.  Min.  Pet. 
Mitth.,  14,  p.  347,  1895).  Working  with  granites,  he  found  that  the  coarser 
portion  of  the  powder  was  higher  in  silica  than  the  finer,  due  to  the  greater 
hardness  (and  less  brittleness)  of  the  quartz.  See  also  I.  A.  Williams,  Amer. 
Geol.,  36,  p.  89,  1905. 

1 1  have  not  succeeded  in  getting  any  indications  of  the  presence  of  metallic 
iron  by  grinding  under  water  in  an  agate  mortar,  as  described  by  Hillebrand 
(p.  51,  note)  the  powders  obtained  by  my  method.  The  samples  (of  rocks  that 
had  been  previously  analyzed)  included  various  kinds;  granite,  diorite,  norite, 
trachyte,  phonolite,  andesite,  and  basalt. 


68  THE  SAMPLE 

involves  a  negligible  loss  of  dust.  This  can  also  be  entirely  done 
away  with  if  the  double  glass  box  be  used  (p.  45).  The  grind- 
ing of  the  small  amount  of  coarse,  final  portion  in  agate  takes  but  a 
short  time,  not  five  minutes  in  all. 

The  objection  against  the  use  of  silk  bolting  cloth  1  is  of  little 
moment.  The  danger  of  contamination  by  particles  of  silk,  and 
hence  of  error  in  the  determination  of  FeO,  is  more  theoretical 
than  real,  and  is  certainly  less  than  the  errors  inherent  in  the 
determination  itself.  If  the  powder  from  the  mortar  be  only 
shaken,  not  rubbed,  through  the  sieve,  only  a  negligible  amount  of 
silk  in  all  would  pass  into  and  would  be  distributed  through  twenty 
or  more  grams  of  rock  powder,  of  which  but  one-half  a  gram  is 
taken  for  the  FeO  determination.  It  is  certain  that  the  reducing 
action  of  the  extremely  small  amount  of  organic  matter  thus  pos- 
sibly present  would  be  very  much  less  than  that  necessary  to  decol- 
orize a  single  drop  of  the  permanganate  solution  used,  and  hence 
would  be  entirely  negligible,  even  for  the  most  accurate  work. 

Metal  sieves  should  not  be  used  in  the  analysis  of  rocks  or 
minerals,  as  they  might  introduce  serious  contamination  or  com- 
plication in  accurate  work. 

It  is,  therefore,  held  that  the  third  method  is  the  one  least  liable 
to  error  and  the  one  which  will  presumably  furnish  material  for  the 
analysis  that  will  more  closely  represent  the  original  rock  than  the 
second,  and  still  more  than  the  first.  It  may  now  be  described  in 
detail. 

Pulverization  of  the  Sample. — The  whole  amount  of  the  sample 
which  is  representative  of  the  rock-mass  is  reduced  to  small  frag- 
ments, if  the  amount  of  material  is  small,  by  breaking  up  with  a 
hardened  hammer  on  the  top  of  the  steel  pestle  which  is  placed 
in  position  in  the  mortar.  Care  must  be  taken  to  avoid  the  flying 
off  of  fragments.2  If  the  pieces  of  rock  are  broken  on  the  pestle- 
head  they  can  be  held  in  the  dry  fingers  and  cracked  by  a  sharp, 
quick  blow,  and  the  pieces  so  obtained  cracked  again.  The  largest 

1  In  Bull.  422,  p.  51,  Hillebrand  advocates  its  use  in  sifting,  if  this  be  neces- 
sary. 

2  Wrapping  the  rock  in  paper  for  the  first  breaking  up,  as  is  sometimes 
done,  is  not  to  be  recommended,  as  it  is  almost  impossible  to  free  the  frag- 
ments entirely  from  adhering  paper,  and  the  considerable  organic  matter  thus 
introduced  will  lead  to  serious  error,  especially  in  the  determinations  of  FeO 
and  H2O, 


PREPARATION  OF  THE  SAMPLE  69 

of  the  fragments  finally  obtained  must  be  small  enough  to  drop 
easily  into  the  mortar,  and  all  of  them,  with  any  resulting  small 
grains  and  powder,  are  placed  on  a  clean  sheet  of  white,  smooth 
paper. 

One  of  the  small  fragments  of  rock  is  then  placed  in  the  steel 
mortar,  which  rests  on  a  firm,  solid  support,  preferably  an  upright, 
solid  block  of  wood,  resting  on  the  floor,  and  is  partially  crushed 
by  a  dozen  or  so  sharp  but  gentle  blows  of  a  light  (one-half  pound) 
hammer.  The  pestle  is  removed  and  placed  on  the  sheet  of  paper, 
and  the  contents  of  the  mortar  are  dropped  into  the  glass  box, 
from  which  the  gauze  and  brass  ring  have  been  removed.  A  few 
gentle  taps  of  the  base  of  the  mortar  against  the  cylinder  assist 
in  removing  the  last  portions  of  adhering  powder  from  the  cavity. 
It  is  well  to  break  up  any  coherent  lumps  of  fine  powder  in  the  glass 
box  by  gentle  pressure  with  the  pestle,  as  this  will  aid  in  the  sub- 
sequent sifting. 

It  is  absolutely  essential,  as  pointed  out  above,  that  there  should 
be  no  rubbing  or  grinding  motion  during  the  crushing.  The 
pestle  and  cylinder  are  held  firmly  in  the  fingers  of  the  left  hand, 
so  that  there  may  be  no  rotary  motion,  and  the  blows  of  the  ham- 
mer should  be  gentle  and  strictly  vertical.  Any  grinding  of 
the  pestle  against  the  sides  of  the  cylinder  or  rotation  on  the 
rock  powder  will  surely  introduce  some  iron  through  the  wearing 
of  the  metal  surface. 

The  whole  of  the  fragments  and  powder  resulting  from  the 
first  crushing  are  to  be  thus  passed  through  the  mortar  and  placed 
in  the  box.  The  cylinder  should  not  be  filled  more  than  to  the 
depth  of  from  3  to  5  mm.  at  a  time,  and  it  is  not  necessary  nor 
advisable  to  crush  all  of  the  rock  to  a  fine  powder  at  this  stage. 
Care  should  be  taken  that  the  cylinder  is  placed  vertically  in  the 
base  before  any  fresh  material  is  placed  in  it,  and  that  the  pestle 
is  also  inserted  in  a  strictly  vertical  position.  Lack  of  attention 
to  these  points  gives  rise  to  the  danger  of  small  shavings  or  chips 
of  steel  being  cut  off  and  falling  into  the  rock  powder. 

When  all  the  sample  taken  has  been  thus  partially  and 
coarsely  pulverized  and  placed  in  the  glass  box,  a  piece  of 
the  silk  gauze,  about  10  or  12  cm.  square,  is  stretched  over 
its  mouth  and  held  firmly  in  place  by  the  brass  ring  which  is 
slipped  over  it.  If  the  double  form  is  used,  the  second  box  is  also 


70  THE  SAMPLE 

put  in  place.  The  sieve  is  then  held  upside  down  over  another 
sheet  of  white,  hard-calendered  paper,1  about  300  by  400  cm. 
(12X16  inches),  a  short  distance  above  it,  and  gently  shaken  from 
side  to  side.  The  paper  is,  of  course,  not  needed  if  the  double  box 
is  used.  This  operation  should  be  conducted  as  gently  as  is  con- 
sistent with  proper  efficiency,  and  in  a  place  free  from  draughts, 
so  as  to  avoid  loss  of  dust. 

When  no  more  powder  falls  through,  the  brass  ring  and  the 
gauze  are  removed,  and  the  contents  of  the  box  are  poured  out  on 
the  first  sheet  of  paper.  The  whole  process  of  crushing  in  the 
steel  mortar  is  then  gone  through  with  on  this  material,  exactly  as 
before,  and  it  is  again  sifted.  The  residue  from  the  second  sifting  is 
again  treated,  and  if  necessary  the  process  is  repeated  till  only  a 
small  amount  (1  or  2  grams)  of  coarse  powder  is  left  in  the  glass  box, 
too  coarse  to  pass  through  the  gauze.  This  will  ordinarily  take 
about  three  or  four  successive  crushings.  This  final  portion  is 
then  ground  down  by  hand  in  the  agate  mortar  in  small  portions 
at  a  time,  the  different  portions  as  they  are  ground  being  scattered 
over  different  parts  of  the  low  heap  of  powder  on  the  sheet  of 
paper,  or  in  the  second  box.  Unless  the  amount  of  material  to  be 
crushed  is  very  large,  or  the  rock  extremely  tough,  three  or  four 
successive  crushings  will  be  all  that  will  be  needed.  The  final 
grinding  of  the  last  small  lot  of  powder  should  never  be  omitted, 
as  this  consists  of  the  tougher  minerals  of  the  rock,  and  if  it  were 
thrown  away,  the  correspondence  between  the  sample  and  the 
rock  would  be  incomplete. 

When  the  whole  is  thus  passed  through  the  sieve,  the  powder  is 
very  thoroughly  mixed.  This  is  best  accomplished  on  the  paper 
by  tilting  up  successively  the  ends  and  the  sides  of  the  paper  until 
the  mass  is  in  the  center.  One  end  of  the  sheet  is  then  raised 
gently  until  the  heap  of  powder  is  lifted  and  turned  over  and  slid 
toward  the  other  end.  It  is  essential  to  proper  mixing  that  the 
mass  of  powder  should  not  only  slide  down,  but  that  it  should 
actually  be  turned  over.  This  is  repeated  many  times,  not  only 
from  end  to  end  but  from  cide  to  side,  with  an  occasional  oblique 
roll.  A  platinum  spatula  may  also  be  used  to  mix  the  powder, 
care  being  taken  that  none  of  the  paper  surface  be  rubbed  off,  but 
the  process  described  above  is  to  be  preferred.  When  it  is  con- 
1  The  sheets  used  by  botanists  for  herbaria  will  be  found  convenient. 


PREPARATION  OF  THE  SAMPLE  71 

sidered  that  the  powder  is  thoroughly  mixed,  it  is  not  an  undue 
precaution  to  roll  it  over  in  different  directions  several  times  more. 
The  powder  may  also  be  quickly  and  thoroughly  mixed  by  putting 
it  again  in  the  box  and  sifting  it  through  a  somewhat  coarser 
gauze,  best  into  the  second  box. 

After  thorough  mixing,  the  powder  is  poured  into  a  specimen 
tube.  For  amounts  of  20  to  30  grams,  one  6  X 1  or  5  X  f  inches 
will  answer,  while  one  of  4  X  \  inches  will  hold  about  10  grams  of 
rock  powder.  The  tube  used  must  be  carefully  cleaned,  inside 
and  out,  by  washing  first  with  distilled  water,  then  with  a  little 
alcohol,  and  thoroughly  dried.  This  is  accomplished  by  the 
application  of  a  gentle  heat,  the  moist  air  being  at  the  same  time 
sucked  out  through  a  piece  of  glass  tubing  attached  to  a  suction- 
pump.  The  tube  must  be  perfectly  cool  before  the  powder  is 
introduced,  and  is  closed  with  a  smooth,  well-fitting  cork,  on  the 
top  of  which  the  number  of  the  specimen  is  written  in  ink.  The 
number,  name,  and  locality  of  the  specimen  should  also  be  written 
on  a  small  label  pasted  on  the  side  of  the  tube. 

In  order  to  avoid  possibility  of  rusting,  the  steel  mortar 
should  be  cleaned,  a  stiff  brush  and  a  dry  cloth  being  used,  as 
soon  as  possible  after  the  preparation  of  the  samples,  and  placed 
in  a  tightly  closed  box.  The  bolting  cloth  -may  be  used  many 
times,  but  it  must  be  very  thoroughly  dusted  free  from  any  trace 
of  rock  powder,  after  each  operation.  For  the  most  accurate  work 
a  fresh  piece  is  to  be  taken.  The  glass  box  or  boxes  and  the  brass 
ring  are  also  to  be  cleaned. 

It  is  of  the  utmost  importance  that  all  of  the  sample  which  is 
prepared  for  the  steel  mortar,  either  the  chips  if  the  amount  of 
material  be  small  or  that  obtained  -by  quartering  if  it  be  large, 
be  pulverized  and  passed  through  the  sieve  or  ground  in  the  agate 
mortar.  If  it  is  only  partially  pulverized  and  the  last  portions  are 
rejected,  it  is  clear  that  the  powder  so  obtained  will  not  represent 
the  average  composition  of  the  rock.  The  rock-forming  minerals 
differ  widely  in  brittleness,  so  that  the  portions  pulverized  first 
will  have  a  content  higher  than  the  average  in  particles  of  the  more 
easily  pulverizable  minerals,  as  quartz,  feldspar  and  feldspathoids, 
while  the  last  portions  will  be  especially  rich  in  the  tougher  min- 
erals, pyroxene,  hornblende  and  the  micas.1  The  micas,  above  all, 

1  Some  data  are  given  by  I.  A.  Williams,  Amer.  Geol.  35,  p.  38,  1905. 


72  THE  SAMPLE 

are  difficult  to  pulverize  completely  either  in  the  steel  or  agate 
mortar,  on  account  of  their  ready  cleavage  and  flexibility,  but  the 
thinness  of  their  flakes  render  these  quite  easy  of  attack  by  the 
reagents  used.  If  they  are  present  in  any  quantity  it  is  necessary 
to  see  that  the  flakes  are  well  distributed  through  the  powder. 

The  analysis  should  always  be  carried  out  on  air-dry  powder, 
as  specially  dried  rock  powder  invariably  reabsorbs  some  or  all 
of  the  lost  moisture  by  exposure  to  the  air  during  the  weighing 
and  whenever  the  tube  is  opened,  or  even  in  time  if  it  be  closed.1 
This  absorption  of  water  is  the  greater  the  finer  the  powder,  and 
the  amount  of  water  thus  absorbed  has  been  shown  by  Day 
and  Allen2  to  be  of  the  same  order  of  magnitude  as  that  of  the 
"hygroscopic"  water  observed  in  the  analyses  of  feldspars  and 
many  rocks  free  from  hydrated  minerals.  The  preparation  of  the 
sample  should,  therefore,  be  undertaken  on  a  dry  day;  never  during 
rain. 

1  Cf.  Hillebrand,  Bull.  422,  p.  65. 

2  Publ.  Carnegie  Inst.,  No.  31,  1905,  p.  57;   and  Am.  J.  Sci.,  19,  1905, 
p.  127. 


PART  IV 

OPERATIONS 

1.  PRELIMINARY  OBSERVATIONS 

IN  the  chemical  laboratory,  above  all  places,  is  "  cleanliness 
next  to  godliness."  The  analyst  must  be  scrupulously  particular 
about  the  freedom  of  the  laboratory  from  dust  and  dirt,  and  about 
the  cleanliness  of  his  apparatus.  No  matter  how  clean  the  labora- 
tory may  be,  all  vessels  whose  contents  are  to  stand  for  more  than 
a  short  time,  and  especially  over  night,  must  be  covered  to  prevent 
the  entrance  of  dust.1  In  prolonged  evaporations  it  is  well  to 
protect  the  liquid  by  a  large  pane  of  glass  or  a  Meyer's  evaporat- 
ing funnel  held  above  the  basin  by  a  clamp  and  support  reserved 
for  this  purpose. 

The  deposit  of  ammonium  salts  that  accumulates  on  the  bot- 
tles standing  on  the  shelves  is  not  only  unsightly,  but  is  a  constant 
possible  source  of  contamination.  The  bottles  should,  therefore, 
be  wiped  off  every  now  and  then,  and  the  work-bench  also  washed 
occasionally. 

Every  glass  utensil  or  other  soiled  piece  of  apparatus  should  be 
washed  clean  and  wiped  dry  immediately  after  using,  and  put  away 
in  its  proper  place.  This  applies  especially  to  beakers.  By  con- 
forming to  this  rule  soiled  vessels  will  not  accumulate,  and  there 
will  be  no  danger  that  they  be  put  away  and  used  inadvertently 
in  place  of  clean  ones. 

A  clean  beaker  is  to  be  used  to  receive  a  filtrate,  even  if  this  is 
to  be  rejected,  so  as  to  permit  the  recovery  of  the  precipitate  if  it 
passes  through  the  filter  or  if  the  latter  breaks.  Before  using  a 
clean  beaker  or  flask,  it  should  be  rinsed  out  with  a  little  distilled 

1  A  beaker  is  covered  with  a  watch-glass,  of  a  size  so  that  the  edge  projects 
about  2  cm.  around  the  beaker,  and  is  placed  convex  side  down.  A  flask  is 
best  covered  with  a  small  beaker,  turned  upside  down. 

73 


74  OPERATIONS 

water  from  the  wash-bottle.  In  volumetric  or  colorimetric  work 
the  burette  should  be  rinsed  out  with  a  little  of  the  solution  to  be 
placed  in  it,  even  if  it  is  apparently  dry. 

The  student  should  shun  all  slovenly  manipulation,  such  as 
spilling  liquids  or  solid  reagents  on  the  work-bench,  or  letting 
liquids  drip  when  pouring  from  the  bottle.  Tobacco  ashes  are  a 
constant  source  of  danger,  though  the  author  must  plead  guilty 
to  running  this  risk  constantly.  The  wearing  of  an  apron  in 
making  a  quantitative  analysis  is  to  be  deprecated,  as  tending  to 
confirm  one  in  slovenly  habits.  If  the  analyst  is  liable  to  drop 
acids  on  his  clothes  he  is  more  than  liable  to  spill  some  of  the 
solutions  he  is  analyzing. 

Every  beaker,  flask,  crucible,  or  other  receptacle  that  contains  a 
precipitate  or  filtrate  obtained  in  the  course  of  the  analysis  and 
that  has  to  be  laid  aside  temporarily  should  be  clearly  labelled 
with  the  number  of  the  analysis  and  the  name  of  the  substance. 
Otherwise  confusion  or  uncertainty  is  almost  sure  to  follow,  espe- 
cially if  a  complicated  analysis  or  a  series  of  analyses  is  in  progress. 
The  labelling  can  be  done  by  placing  a  piece  of  paper  with  the 
requisite  data  on  the  cover  glass  or  by  writing  on  the  spot  of 
ground  glass  on  the  side  of  the  beaker  or  flask. 

The  beginner  has  a  marked  tendency  to  over-carefulness  in 
some  details  of  the  analysis.  This  excess  of  zeal  is  expressed  in 
using  utensils  of  sizes  that  are  much  larger  than  are  needed,  in 
using  inordinate  amounts  of  precipitant,  of  wash  water,  in  igniting 
precipitates  for  a  much  longer  time  than  is  necessary,  or  in  using  a 
much  too  high  Bunsen  flame.  This  tendency  is  so  pronounced 
that  the  beginner  in  analysis  should  be  on  his  guard  against  it. 
It  is  to  be  discouraged  because  it  not  only  greatly  lengthens  the 
time  needed  for  the  analysis  but,  which  is  more  serious,  it  tends 
to  lessen  the  accuracy  of  the  work  through  the  solution  of  precip- 
itates and  for  other  reasons. 

The  beginner  should  take  full  notes  during  the  progress  of 
the  first  analyses,  until  the  various  methods  become  familiar,  and 
even  then  all  occurrences  or  manifestations  out  of  the  ordinary 
are  to  be  noted  and  not  left  to  the  memory.  The  details  of  all  the 
calculations  are  to  be  recorded  in  the  note-book  for  future  refer- 
ence. It  may  sometimes  happen  that  an  apparent  analytical 
error  or  an  unsatisfactory  summation  is  merely  due  to  a  slip  in 


SOURCES  OF  OPERATIVE  ERRORS          75 

arithmetic,  and  a  re-examination  of  the  recorded  weights  and 
calculations  may  obviate  the  necessity  of  a  duplicate  analysis. 

In  rock  analysis  a  preliminary  qualitative  examination  is 
seldom,  if  ever,  necessary.  The  microscope  will  often  serve  the 
purpose.  But  if  not,  and  if  the  presence  of  some  unusual  sub- 
stance is  suspected,  it  is  better,  as  Hillebrand  remarks,  to  assume 
its  presence  and  conduct  the  quantitative  analysis  on  this  assump- 
tion. This  will  be  time  saved  in  the  end,  even  if  the  result  is 
merely  to  prove  the  absence  of  the  suspected  body,  which  in  itself 
may  be  a  fact  of  some  interest.  In  such  cases,  one  should  always 
test  by  qualitative  methods  the  character  of  the  weighed  precipitate 
to  see  whether  it  is  really  the  substance  in  question  or  not. 

Finally,  before  beginning  an  analysis  the  student  should  see 
that  the  balance  is  correctly  adjusted,  and  that  all  the  necessary 
apparatus  and  reagents  are  at  hand,  so  that  the  work  may  pro- 
ceed without  interruption.  It  will  be  well  to  read  the  whole  of 
the  description  of  each  of  the  various  operations  and  methods  be- 
fore beginning  their  execution,  as  some  information  may  be  given 
at  the  end  which  is  essential  to  the  proper  performance.  Thus,  in 
the  determination  of  combined  water,  if  the  rock  which  is  being 
analyzed  contains  haiiyne  or  sodalite,  and  the  whole  description 
of  the  method  has  not  been  read,  the  student  may  be  unaware  of 
the  necessity  for  retaining  the  chlorine  or  sulphur  trioxide,  and  so 
will  obtain  erroneous  results. 

As  illustrative  of  the  precautions  that  must  be  taken  in  highly 
accurate  work,  it  may  be  of  interest  to  the  student  to  read  Rich- 
ards' discussion  of  the  Methods  Used  in  Precise  Chemical  Investi- 
gation.1 

In  subsequent  pages  some  of  the  most  important  analytical 
operations  are  described  in  considerable  detail,  and  it  will  be  well 
for  the  beginner  to  read  them  before,  and  consult  them  during,  the 
making  of  the  analysis. 

2.  SOURCES  OF  OPERATIVE  ERRORS 

The  distinction  between  errors  that  are  incidental  to  operations 
and  those  that  are  inherent  in  the  methods  used  will  be  discussed 
later  (p.  119).  It  may  be  useful  here  to  give  a  list  of  those 

1  T.  W.  Richards,  Determinations  of  Atomic  Weights,  Carnegie  Institution 
Publication,  No.  125,  p.  97,  1910. 


76  OPERATIONS 

sources  of  error  that  are  connected  with  the  various  operations 
which  are  more  serious  in  their  consequences  or  which  may  be  most 
often  met  with  by  the  student,  and  which  should  be  more  particu- 
larly guarded  against.1  The  precautions  that  may  be  taken 
against  some  of  them  are  given  in  connection  with  the  descriptions 
of  the  various  operations. 

This  list  is  so  long  and  formidable  in  appearance  that  the 
beginner  may  be  disheartened  by  the  prospect  of  continuous  vigi- 
lance that  confronts  him.  For  his  encouragement  it  may  be  said 
that  their  aggregate  number  is  more  serious  in  the  seeming  than 
are  in  reality  the  occurrence  or  possible  consequences  of  many  of 
them.  The  only  ones  that  are  really  inexcusable  at  the  begin- 
ner's stage  are  those  arising  from  carelessness,  and  these  may  soon 
be  overcome  by  proper  attention  and  thought.  The  great  major- 
ity of  the  others  will  readily  be  avoided  or  overcome  by  the  atten- 
tive and  conscientious  student,  who,  not  only  follows  the  condi- 
tions and  precautions  mentioned  in  the  various  descriptions  of 
operations  and  methods,  but  who  also  looks  upon  the  analytical 
work  with  intelligent  interest,  and  consequently  does  not  "  learn 
nothing  more  than  to  follow  directions."  To  such  a  worker  the 
very  sources  of  error  themselves  are  of  interest,  stimulating  him 
to  constant  attention,  and  lending  to  the  progress  of  the  analysis 
an  element  of  interest  and  something  akin  to  excitement  that  it 
would  lack  if  every  operation  and  method  were  wholly  free  from 
possible  error. 

Sampling. — Correct  sampling  is  of  fundamental  importance, 
but  this  must  be  considered  as  a  preliminary  to  the  analysis. 

Unfavorable  Conditions. — The  laboratory  may  be  in  a  dusty 
location,  or  not  kept  in  a  properly  neat  and  clean  condition; 
the  laboratory  air  may  be  contaminated  by  fumes;  machinery  in 
motion  may  disturb  the  balance  readings;  the  ceiling  may  be  of 
such  character  or  in  such  condition  as  to  drop  particles  into  vessels 
beneath;  a  window  facing  the  sun,  or  some  other  source  of  heat, 
may  affect  the  balance;  the  exterior  light  surroundings  may  be 
such  (brick  walls  or  foliage)  as  to  disturb  colorimetric  readings; 
the  air  may  be  so  damp  that  powders  or  objects  to  be  weighed 
absorb  a  disturbing  amount  of  moisture*  the  attention  may  be 
distracted  by  interruptions  or  noise. 

1  Mellor  (p.  249)  gives  a  similar  but  shorter  list. 


SOURCES  OF  OPERATIVE  ERRORS          77 

Personal  Equation. — There  may  be  personal  peculiarities  in 
the  reading  of  burettes  and  other  instruments  that  lead  to  con- 
stantly high  or  low  results;  there  may  be  a  peculiarity  in  color 
perception  that  tends  to  constant  high  or  low  estimates  in  colori- 
metric  determinations;  one  eye  may  perceive  colors  differently 
from  the  other;  impatience  may  lead  to  the  undue  hastening  or 
shortening  of  operations;1  overwork  may  cause  fatigue  and 
consequently  lead  to  many  opportunities  for  error;  the  need 
for  quickly  attained  results  may  cause  errors  of  many  kinds;  the 
analyst  may  not  be  in  good  health. 

Apparatus. — The  balance  may  not  be  properly  adjusted;  one 
arm  may  have  been  lengthened  by  heating  from  a  window,  etc. ; 
the  zero  point  may  not  be  known,  and  it  may  be  distant  from  the 
center  of  the  scale;  the  weights  may  not  be  correct;  the  measuring 
flasks  and  burettes  may  not  be  consistent  with  each  other;  the 
glass  of  which  the  beakers  are  made  may  be  too  readily  attacked 
by  reagents. 

Impure  Reagents. — The  impurities  found  in  reagents  are  of 
great  variety,  and  some  of  them  quite  unexpected.  It  is  not  prac- 
ticable or  necessary  to  enumerate  them. 

Standard  Solutions. — The  standard  solutions  of  permanganate, 
titanium,  and  manganese  are  liable  to  change  and  deterioration 
with  lapse  of  time;  they  may  change  by  evaporation  through 
opening  the  bottle  or  pouring  them  out;  their  titer  may  be  in- 
creased by  not  shaking  the  bottle  previous  to  use,  or  lessened 
by  moisture  in  the  burette. 

Numerical  Errors. — The  position  of  the  rider  on  the  beam,  the 
weights,  the  position  of  the  meniscus  in  the  burette  or  that  of  the 
mercury  in  a  thermometer,  may  be  incorrectly  read  or  noted  down ; 
an  incorrect  factor  or  an  incorrect  figure  from  a  table  may  be  used 
in  calculations;  arithmetical  mistakes  may  be  made  in  the  calcu- 
lations; the  summation  of  the  results  of  the  analysis  may  be  in- 
correct. 

General  Sources. — Volumes  may  not  be  corrected  for  temper- 
ature conditions;  platinum  crucibles  may  lose  weight  on  ignition; 
liquids  may  be  contaminated  by  solution  of  the  material  of  the 
containing  vessel;  precipitates  may  be  contaminated  by  other 
substances  that  are  carried  down  with  them  in  the  precipitation; 
1  This  might  come  more  properly  under  the  head  of  carelessness. 


78  OPERATIONS 

liquids  or  solids  may  be  contaminted  by  absorption  of  fumes  or 
gases  (as  carbon  dioxide  or  ammonia)  from  the  atmosphere;  sul- 
phur may  be  introduced  from  the  burner  gas  or  from  rubber  stop- 
pers; platinum  may  be  introduced  during  fusion  in  crucibles  or 
evaporation  in  basins  of  this  metal;  copper  may  be  introduced 
from  copper  utensils. 

Inexperience. — Errors  due  to  this  source  may  be  very  serious, 
but  they  are  generally  less  harmful  than  those  caused  by  careless- 
ness, and  they  should  soon  be  overcome  with  practice  and  atten- 
tion. 

The  most  common  and  persistent  source  of  error  due  to  inex- 
perience is  the  tendency  to  bigness  and  the  overdoing  of  things. 
A  too  large  size  of  vessels,  such  as  beakers,  adds  to  the  difficulties 
of  manipulation,  increases  the  volumes  of  wash  water  and  other 
liquids,  tends  to  the  over-washing  of  precipitates,  and  increases  the 
time  taken  for  the  analysis;  the  use  of  too  large  filters  tends  to 
the  over-washing  of  precipitates  and  the  under-washing  of  the 
filter;  the  volumes  of  wash  liquids  or  the  amounts  of  precipitants 
are  easily  made  much  larger  than  is  needed  or  desirable,  and  this 
greatly  increases  the  chances  of  error  in  many  ways;  the  use  of 
too  large  flames  may  lead  to  reduction  of  the  substance  ignited  or 
loss  by  draughts,  and  it  also  wastes  gas  and  makes  unnecessary 
noise;  the  over- washing  of  precipitates  leads  to  their  partial 
solution  and  to  too  low  results ;  too  prolonged  ignition  may  cause 
undue  loss  of  weight  of  the  crucible,  change  or  loss  of  weight  of  the 
ignited  substance,  and  always  loses  time. 

Insufficient  washing,  or  cessation  of  ignition  before  constant 
weight  has  been  attained,  lead  to  too  high  results;  improper  posi- 
tion of  the  crucible  as  regards  the  flame,  especially  that  of  a  blast, 
may  cause  loss;  the  loss  of  molten  substances,  such  as  of  the 
carbonate  fusion,  by  spattering,  or  of  the  pyrosulphate  fusion  by 
spattering  or  by  "  creeping,"  will  cause  low  results. 

Among  other  sources  of  error  are  the  reading  of  a  burette 
before  all  the  liquid  has  run  down  the  wall ;  the  loss  of  precipitate 
by  "  creeping  "  up  the  sides  of  a  beaker  or  funnel;  the  incom- 
plete removal  of  a  precipitate  from  the  sides  of  a  beaker;  effer- 
vescence on  the  addition  of  acids  to  carbonates  (as  in  the  solution 
of  the  carbonate  cake)  or  that  of  strong  ammonia  water  to  hot  or 
acid  liquids;  incomplete  reduction  of  ferric  oxide  by  not  passing 


WEIGHING  79 

hydrogen  sulphide  for  a  sufficient  time  or  the  partial  oxidation  of 
ferrous  oxide  by  faulty  manipulation;  the  throwing  out  of  precip- 
itate by  incautious  application  of  the  jet;  and  an  improper  man- 
ner of  introducing  reagents  or  precipitants. 

Carelessness. — The  errors  due  to  this  cause  are  the  least  excus- 
able and  are  generally  the  most  serious  as  well  as  the  most  easily 
avoided.  They  include:  Injury  to  the  balance  or  weights  by 
improper  handling;  placing  hot  or  damp  objects  on  the  pan; 
the  spilling  of  liquids  or  powders  during  transfer;  the  loss  of  liquid 
by  splashing  or  "  bumping,"  or  of  powder  by  draughts  of  air  or 
by  their  "  puffing  up  "  on  the  addition  of  liquids;  the  use  of  dirty 
utensils  and  an  untidy  or  dirty  work-bench;  the  dropping  or 
breaking  of  vessels;  the  ill-treatment  of  platinum  crucibles,  so 
that  they  become  dented  or  otherwise  injured;  laying  the  stoppers 
of  reagent  bottles  on  an  unclean  surface,  such  as  that  of  the  work- 
bench; the  absence  of  a  cover  glass  on  a  beaker  during  boiling,  or 
that  of  the  crucible  cover  during  a  fusion  or  ignition ;  the  incorrect 
labelling  or  not  labelling  the  various  precipitates  and  filtrates; 
inattention  to  operations  in  progress,  not  washing  used  vessels 
as  soon  as  possible  and  putting  them  in  their  proper  places;  and 
not  following  the  given  directions  closely. 

3.    WEIGHING1 

Preliminary  Remarks. — The  object  to  be  weighed  should  be 
perfectly  dry,  so  far  as  its  surface  goes/  as  damp  objects  will  not 
only  give  incorrect  results  but  will  injure  the  pan. 

The  object  should  always  be  at  the  temperature  of  the  room. 
Nothing  hot,  or  even  warm,  is  ever  to  be  placed  on  the  balance 
pans.  Apart  from  possible  injury  to  and  staining  of  the  pan,  the 
air  currents  set  up  by  a  warm  object  may  buoy  up,  so  to  speak, 
the  arm  of  the  beam,  thus  making  the  weight  apparently  less  than 
it  really  is;  or  the  heated  air  may  expand  the  arm  and  thus  give 
an  apparently  too  great  weight.  On  the  other  hand  the  object 
should  not  be  colder  than  the  surrounding  air,  because  of  the  liabil- 
ity to  the  condensation  of  moisture  on  it. 

No  solid  reagent  of  any  kind,  whether  in  the  form  of  powder 

1Fresenius,  1,  pp.  21-25;  Gooch,  pp.  14-24-  Mellor,  pp.  7-27'  Morse, 
pp.  8-22;  Treadwell,  2,  pp.  8-1$, 


80  OPERATIONS 

or  not,  is  to  be  placed  directly  on  the  pan.  Such  substances  should 
always  be  placed  in  and  weighed  with  some  receptacle,  such  as  a 
crucible,  watch  glass,  or  weighing  tube  or  bottle. 

The  object  to  be  weighed  should  always  be  placed  (for  a  right- 
handed  person)  on  the  left-hand  pan,  and  the  weights  on  the  right- 
hand  pan.  This  rule  is  to  be  strictly  adhered  to,  as  it  eliminates 
error  due  to  inequality  of  the  balance  arms  (Mellor,  p.  22). 

When  weighing  a  crucible  that  weighs,  for  example,  19  grams, 
it  is  a  great  temptation,  in  order  to  save  time  and  labor,  to  put  the 
20-gram  weight  on  the  right-hand  pan  and  a  1-gram  weight  on  the 
left,  with  the  crucible.  Although  this  procedure  may  be  adopted 
by  the  experienced  analyst,  who  knows  his  crucibles,  and  for  the 
gram  weights  only,  it  should  be  avoided  by  the  student,  as  it  is 
very  likely  to  cause  confusion  and  error  in  noting  the  weights. 
Above  all,  it  should  never  be  done  with  the  decigram  and  centigram 
weights,  where  the  practice  is  almost  sure  to  cause  error. 

The  object  to  be  weighed,  as  well  as  the  heavier  weights,  should 
be  placed  at  the  center  of  the  left-  and  right-hand  pan,  respectively. 
This  will  prevent  the  sometimes  violent  swinging  of  the  pan  that 
occurs  when  a  heavy  mass  is  placed  near  its  edge,  and  which  may 
be  difficult  to  stop.  Large  and  irregularly  shaped  objects,  such  as 
a  weighing  burette,  are  to  be  suspended  from  the  hook  above  the 
pan,  and  a  tube  is  best  supported  on  a  light  metal  frame  to  prevent 
rolling. 

The  pans  should  not  be  allowed  to  have  any  rotary  or  swinging 
motion,  as  this  may  cause  injury  to  the  knife  edges.  If  either  pan 
swings  on  release,  the  motion  should  be  stopped  by  gently  raising 
the  pan  arrest  so  as  to  stop  the  pan  at  the  middle  of  the  swing. 
On  full  release,  during  the  observation  of  the  pointer,  the  pans 
should  hang  vertically  and  with  no  swinging  from  side  to  side. 

The  weights  and  crucibles,  or  such  objects,  should  be  handled 
only  with  the  ivory-tipped  forceps,  and  should  be  placed  on  and 
removed  from  the  pans  very  gently.  Indeed,  all  motions  involved 
in  weighing  should  be  gentle  and  slow.  No  weight,  not  even  a 
centigram  one,  is  to  be  dropped  on  the  pan. 

Both  the  beam  and  pans  are  to  be  arrested  before  placing  any- 
thing on,  or  removing  anything  from,  the  pans.  The  motion  in 
arrest  and  release  should  be  very  gentle  and  gradual,  and  the  beam 
should  be  arrested  only  when  the  pointer  is  at,  or  at  most  not  more 


WEIGHING  81 

than  one  division  from,  the  zero  point.  The  balance  should  never 
"  chatter  "  through  arresting  the  beam  too  quickly.  These  rules 
are  very  important,  and  neglect  of  them  will  inevitably  lead  to 
speedy  impairment  of  the  balance's  accuracy. 

The  balance  case  is  left  open  while  putting  on  the  weights  down 
to,  and  including,  the  lowest  needed  from  the  box,  but  is  to  be 
closed  during  the  weighing  with  the  rider  for  milligrams  and  tenths. 

The  observer  should  sit  directly  in  front  of  the  center  of  the 
balance  "  so  as  to  avoid  errors  due  to  parallax  in  reading  the 
pointer."  It  should  be  seen  that  the  balance  is  level  and,  if  neces- 
sary, it  is  to  be  adjusted  to  horizontality  by  the  levelling  screws 
and  the  plumb-bob  or  spirit-level  that  should  be  part  of  the  bal- 
ance case  equipment.  The  rider,  supported  on  its  hook,  should  be 
so  far  above  the  beam  that  there  is  no  possibility  of  the  beam  hit- 
ting it. 

If  the  zero-point  has  not  been  recently  taken,  or  if  it  is  unknown, 
it  should  be  determined  before  a  series  of  weighings.  It  is  espe- 
cially important  to  determine  the  zero-point  if  the  balance  is  at  the 
disposal  of  several  persons  or  is  in  general  use.  Carelessness  with 
the  balance  is  far  too  frequent. 

To  determine  the  zero-point,  the  beam  of  the  empty  balance  is 
gently  released,  and  if  it  does  not  swing  it  is  set  to  swinging  slightly 
by  a  little  puff  of  air  from  the  rubber  bulb  (p.  29),  or  by  waving 
the  hand  gently  near  one  of  the  pans,  great  care  being  taken  not 
to  touch  this.  The  swings  should  not  be  more  than  ten  divisions 
on  either  side  of  the  center  of  the  scale.  The  balance  case  is  then 
closed  and,  after  two  or  three  swings,  the  divisions  on  either  side 
of  the  center  at  which  the  pointer  stops  and  turns  back  are  recorded, 
tenths  of  a  division  being  estimated,  best  with  the  reading  glass. 
Note  two  or  three  swings  to  the  left  and  a  number  one  greater  than 
this  to  the  right.  Take  the  mean  of  the  left  and  right  swings. 
Add  these  together,  divide  by  two,  and  subtract  the  quotient  from 
the  greater  of  the  two  mean  swings.  The  difference  is  the  zero- 
point,  that  is,  the  distance  from  the  center  of  the  scale  at  which 
the  pointer  would  stop,  either  right  or  left,  if  the  beam  were  allowed 
to  come  to  rest. 

If  the  zero-point  is  within  one  division  of  the  center  on  either 
side,  the  weighing  may  be  carried  out  on  the  assumption  that  the 
zero-point  is  the  center  of  the  scale,  as  the  balance  should  be  so 


82  OPERATIONS 

adjusted  that  one  division  corresponds  to  one-tenth  of  a  milligram, 
which  is  the  limit  of  weighing.  If  it  amounts  to  one  or  two  divisions 
this  zero-point  is  to  be  taken  as  the  center-point  of  the  swings  in 
weighing.  If  it  is  three  or  more  divisions  from  the  center,  and  if 
the  balance  has  been  dusted  carefully,  the  balance  should  be 
adjusted  so  as  to  bring  the  zero-point  near  the  center  of  the  scale 
by  very  cautiously  moving  one  of  the  screw  weights  at  the  ends  of 
the  arms. 

In  ordinary  analytical  work  there  is  no  need  of  adopting  such 
refinements  as  correcting  for  the  buoyancy  of  the  air,  or  such 
accurate,  but  time-consuming,  methods  of  weighing  as  those  of 
Gauss  or  Borda.  The  usual  method  by  swings  is  used. 

Process  of  Weighing. — The  object  to  be  weighed,  if  it  is  a 
crucible,  should  be  ignited  at  a  bright  red  heat  for  a  few  minutes, 
placed  in  the  desiccator  when  it  has  cooled  to  below  visible  redness 
(never  when  red-hot),  and  allowed  to  cool  to  room  temperature 
before  being  weighed.  This  will  take  ten  minutes  or  so  with  plat- 
inum but  much  longer  with  porcelain.  The  object  of  the  ignition 
is  to  bring  the  crucible  into  the  same  condition  that  it  will  be  in 
after  the  ignition  of  the  precipitate.  If  the  object  cannot  be 
ignited,  but  is  dried  in  the  oven,  it  is  placed  in  the  desiccator  imme- 
diately after  removal  from  the  oven  and  allowed  to  cool  to  room 
temperature.  A  specimen  tube,  weighing  burette,  weighing  bottle, 
or  absorption  tube,  is  to  be  wiped  with  a  dry  and  clean  cloth. 
Such  objects  may  be  handled  lightly  with  the  tips  of  the  dry 
fingers,  which  will  remove  any  electrification  caused  by  the 
rubbing. 

After  having  made  sure  that  the  pans  and  beam  are  arrested, 
the  object  to  be  weighed,  say  a  crucible,  is  placed  on  the  center 
of  the  left-hand  pan,  and  weights  approximately  equal  to  that 
of  the  crucible  l  on  the  right-hand  pan.  The  pans  are  released 
very  gently  and  if  they  swing  they  are  to  be  stopped  as  described 
above.  Then  the  beam  is  very  slowly  and  slightly  released,  the 
direction  of  movement  of  the  pointer  is  noted,  and  the  beam  and 
pans  are  again  very  gently  arrested.  If  the  weight  on  the  right- 
hand  pan  is  too  great,  the  smallest  weight  is  removed,  or  if  it  is  too 
small  the  next  larger  weight  is  added ;  then  the  pans  and  beam  are 

1  It  is  convenient  to  have  in  the  balance  case  a  small  card  on  which  are 
written  the  numbers  and  weights  of  the  different  crucibles  in  use. 


WEIGHING  83 

released  and  the  pointer  is  observed  as  before.  The  process  is 
continued  in  this  way,  adding  or  removing  the  weights  systemat- 
ically,1 and  not  at  random,  until  no  more  weights  need  be  put  on 
the  pan,  and  until  10  milligrams,  measured  with  the  rider  on 
the  beam,  moves  the  pointer  to  the  left,  in  other  words,  is  too 
much. 

The  balance  case  is  now  closed  and  the  weighing  is  finished  with 
the  rider.  This  should  also  be  carried  out  systematically;  for 
example,  by  beginning  with  the  rider  at  the  center  of  the  beam  and 
moving  it  successively  smaller  distances.  When  the  weighing 
gets  down  to  tenths  of  a  milligram  (small  beam  divisions)  the 
extreme  excursions  of  the  pointer  are  to  be  noted.  The  tenth- 
milligram  is  regarded  as  final  that  gives  a  zero-point  the  same  as 
that  of  the  empty  balance,  or  when  the  swings  are  equal  on  both 
sides  of  the  center  of  the  scale  if  the  zero-point  is  only  one  or  two 
divisions  away. 

The  beam  is  swung  or  set  in  motion  during  these  final  obser- 
vations by  using  the  rubber  bulb  or  by  lightly  waving  the  hand  up 
and  down  near  one  pan,  taking  care  not  to  touch  this.2 

In  entering  the  weights  in  the  note-book,  it  is  best  done  sys- 
tematically and  uniformly,  by  first  noting  down  the  weights  from 
the  vacant  spaces  in  the  weight-box,  and  then  checking  up  as  the 
weights  are  removed  from  the  pan  arid  replaced  in  their  proper 
places  in  the  box. 

As  Fresenius  points  out3 :  "  The  student  should  from  the  com- 
mencement make  it  a  rule  to  enter  the  number  to  be  deducted 
in  the  lower  line."  The  entry  in  the  note  book  should,  therefore, 
be  in  this  form: 

Cruc. +subst.  =  33 . 0909 
Cruc.  =32.0712 


1.0197 

1  The  weights  should  not  be  placed  one  on  the  other,  but  should  be  laid  down 
alongside  each  other  and  in  order  from  heavier  to  lighter;  this  will  facilitate 
the  checking  up  of  the  weight  and  diminish  the  possibility  of  error  in  replacing 
the  weights  in  their  proper  places  in  the  box. 

2  The  beam  can  also  be  set  in  motion  by  placing  the  rider  on  the  beam  for 
an  instant,  or  by  a  "  trick  "  in  releasing  it;  but  these  should  not  be  tried  by 
the  beginner. 

3  Fresenius  I.,  p.  21, 


84  OPERATIONS 

The  uniform  adoption  of  this  procedure  will  lessen  the  possibility 
of  annoying  arithmetical  mistakes. 

When  the  weighing  is  ended,  it  should  be  seen  that  the  pans 
are  empty  and  free  from  dust  or  specks,  the  balance  case  is  closed, 
the  rider  lifted  from  the  beam,  and  the  weight-box  closed,  so  that 
all  will  be  safe  from  dust  and  immediately  ready  for  another 
weighing.  The  practice  of  leaving  weights  on  the  pan,  as  during 
the  ignition  of  a  precipitate,  which  is  simply  to  save  the  slight 
trouble  of  taking  them  off  and  putting  them  on  again,  is  very  inju- 
dicious. It  is  one  of  the  small  things  that  marks  a  person  who  does 
not  appreciate  the  fundamental  importance  of  the  balance. 

A  word  may  be  said  of  the  practice  of  so-called  "  rational  " 
weighing;  that  is,  the  weighing  out  of  an  exact  amount  of  sub- 
stance so  that  the  weight  of  a  precipitate  will  express  immediately 
the  percentage  of  a  constituent.  Thus,  if  exactly  0.5308  gram  of 
a  substance  that  contains  Na2O  but  no  E^O  is  weighed  out,  the 
weight  of  the  NaCl  obtained  will  be  the  percentage  of  the  Na2O. 
This  is  because  the  weight  of  the  NaCl  is  multiplied  by  .5308  to 
reduce  it  to  Na2O,  and  dividing  the  product  by  .5308  (the  weight 
of  substance  taken)  to  get  the  percentage  will,  of  course,  yield  a 
quotient  identical  with  the  weight  of  NaCl.  An  example  will  be 
found  on  p.  246. 

The  only  purpose  of  such  weighing  is  to  avoid  calculation,  and 
generally  laziness  here  defeats  its  own  object.  The  mental  labor 
involved  in  the  calculation  is  very  slight,  indeed  less  than  that 
involved  in  the  exact  weighing,  while  the  latter  generally  consumes 
far  more  time.  For  both  reasons,  but  especially  because  of  the 
loss  of  time,  the  practice  is  not  commended.1 

4.    DECOMPOSITION2 

After  the  portion  of  rock  powder  is  weighed  out  it  must  be 
decomposed,  either  by  fusion  with  a  flux  or  treatment  with  acid, 
so  as  to  bring  it  into  solution  preliminary  to  analysis. 

1  Cf .  Mellor,  p.  54. 

2  A  mild  protest  may  here  be  registered  against  the  phrase  "  opening  up," 
used  by  some  English  chemists,  instead  of  "  decomposition."     The  former 
suggests  rather  getting  at  the  contents  of  a  tin  of  sardines  or  removing  an 
obstinate  oyster  from  its  shell  than  getting  a  mineral  into  proper  condition  for 
analysis. 


DECOMPOSITION  85 

A  number  of  minerals,  such  as  leucite,  nephelite,  and  olivine, 
are  easily  and  completely  decomposed  by  hydrochloric  acid,  and 
their  analysis  may  be  effected  after  such  a  simple  preliminary 
solution.  Others  again,  such  as  quartz,  orthoclase,  albite, 
pyroxene,  and  hornblende,  are  either  quite  unattacked  or  only 
partially  decomposed  by  this  medium.  Since  practically  no 
igneous  rocks,  so  far  as  we  know,  are  composed  entirely  of  the  first 
class  of  minerals  and  are  completely  soluble  in  hydrochloric  acid, 
it  is  necessary  to  bring  their  constituents  into  soluble  form  by 
other  means,  as  a  preliminary  to  their  analysis. 

A  number  of  methods  have  been  proposed  for  this  purpose, 
some  of  them  based  on  the  use  of  hydrochloric,  sulphuric  or 
hydrofluoric  acids,  and  others  involving  the  use  of  various  fluxes, 
as  alkali  hydroxides,  carbonates  or  fluorides,  calcium  carbonate, 
lead  or  bismuth  oxide  and  boric  acid.  A  description  and  dis- 
cussion of  these  is  given  by  the  authors  cited  below,1  but  it  is 
unnecessary  to  enter  into  this  phase  of  the  matter  here.  It  will 
suffice  to  describe  only  those  methods  which  commend  themselves" 
to  the  author  and  to  the  chemists  of  the  U.  S.  Geological  Survey, 
and  the  use  of  which  is  recommended  in  this  book. 

In  order  to  determine  the  different  constituents  of  a  rock,  dif- 
ferent reagents  and  methods  of  decomposition  are  found  to  be 
appropriate,  depending  on  the  constituents  to  be  determined  in  a 
given  portion.  Those  with  which  we  shall  have  to  deal  are: 

1.  Fusion  with  sodium  carbonate,  for  all  the  main  constit- 
uents (except  ferrous  oxide  and  alkalies),  and  also  for  zirconia, 
baryta,  etc. 

2.  Fusion  with  calcium  carbonate  and  ammonium  chloride, 
for  the  alkalies. 

3.  Fusion  with  potassium  pyrosulphate,  for  total  iron  oxides. 

4.  Solution  in  a  mixture  of  sulphuric  and  hydrofluoric  acids, 
for  ferrous  oxide  and  for  manganous  oxide. 

5.  Solution  in  a  mixture  of  nitric  and  hydrofluoric  acids,  for 
phosphorus  pentoxide. 

6.  Digestion  with  hydrochloric  acid  for  sulphur  trioxide. 

7.  Digestion  with  nitric  acid  for  chlorine. 

The  reagents  employed,   and  consequently  the  methods  of 

^resenius,  1,  pp.  511-521;  Hillebrand,  pp.  83-90;  Mellor,  pp.  160-166; 
Morse,  pp.  310-318;  Treadwell,  pp.  485-491. 


86  OPERATIONS 

procedure,  vary  so  much  that  few  general  rules  can  be  laid  down, 
and  the  special  precautions  to  be  observed  in  each  case  will  be 
described  in  their  proper  places  later.  A  few  general  suggestions 
may  be  given  here. 

In  a  fusion  with  a  powdered  flux,  such  as  sodium  carbonate,  the 
rock  powder  and  the  flux  should  be  well  mixed.  This  may  be  done 
with  a  platinum  spatula,  and  especial  care  should  be  taken  that  the 
flux  is  brought  well  down  to  the  bottom  and  around  all  the  corners 
of  the  crucible,  so  that  no  patches  of  unmixed  rock  powder  are 
left  at  the  bottom.  After  smoothing  down  the  surface,  the  spatula 
is  to  be  cleaned  off  by  rubbing  it  on  a  little  flux  left  on  the  watch- 
glass,  which  is  added  to  that  in  the  crucible. 

The  mixing  of  calcium  carbonate  and  ammonium  chloride 
with  the  rock  powder  must  be  so  intimate  that  it  demands  special 
precautions,  which  will  be  found  described  on  p.  196. 

In  adding  any  flux  to  a  rock  powder  care  should  be  taken  that 
none  of  the  rock  powder  flies  up  and  is  lost. 

The  heating  of  the  mixture  of  flux  and  rock  powder  should  begin 
very  gradually,  the  crucible  at  least  10  cm.  above  a  rather  low 
Bunsen  flame  to  drive  off  moisture.  After  five  or  ten  minutes 
it  is  lowered  until  the  bottom  is  faintly  red ;  it  is  kept  thus  another 
five  to  ten  minutes,  and  then  the  flame  is  gradually  increased 
until  the  mass  sinters  and  finally  fuses  quietly.  When  the  mass 
is  in  fusion  the  height  of  the  flame  and  of  the  crucible  above  it  are 
to  be  so  adjusted  that,  although  the  mass  is  in  full  fusion,  there  is 
no  spattering  onto  the  crucible  cover,  which  should  be  kept  on 
during  the  whole  operation.  This  will  demand  some  attention  at 
first  from  time  to  time,  but  with  practice  the  right  conditions  are 
soon  learned.  Above  all,  spattering  must  not  be  allowed  to  take 
place,  even  at  the  expense  of  greater  time  for  the  fusion. 

After  coming  to  a  state  of  quiet  fusion  at  a  low  red  heat,  the 
mass  should  be  kept  so  for  at  least  ten  minutes  to  insure  complete 
decomposition.  Blasting  is  not  necessary  with  most  rocks.  The 
liquid  will  seldom  be  perfectly  clear  and  transparent,  as  the  car- 
bonates of  iron,  magnesium  and  calcium  will  form  cloudy  masses 
within  it,  so  that  any  such  appearances  need  cause  no  concern. 
Indeed,  with  very  femic  rocks  the  mass  may  seem  to  be  com- 
pletely fused  only  around  the  edges,  owing  to  the  abundance  of 
these  substances,  although  the  rock  is  completely  decomposed. 


PRECIPITATION  87 

A  little  sodium  carbonate  may  vaporize  and  condense  on  the 
under  side  of  the  cover,  but  this  is  of  no  importance. 

The  method  of  loosening  the  solid  cake  after  fusion  is  described 
on  p.  136. 

In  dissolving  rock  powders  in  acids,  two  points  are  to  be  spe- 
cially attended  to.  The  first  is  the  liability  of  fine  powders  to  fly 
off  when  mixed  with  liquid.  This  can  be  avoided  by  carefully 
dropping  a  very  little  water  in  so  as  to  moisten  the  powder,  before 
adding  the  acid.  The  tip  of  the  wash-bottle  should  be  full  of 
water  so  as  to  cause  no  puff  of  air,  and  it  should  be  inserted  be- 
neath the  slightly  raised  crucible  cover,  which  is  lowered  into 
place  immediately.  The  second  point  is  that  here,  also,  there 
should  be  no  spattering,  so  that  the  heating  should  be  cautious 
and  gradual  at  first,  and  the  liquid  should  never  actually  boil  or 
bubble. 

All  evaporations  and  digestions  in  which  the  vapors  of  strong 
acids  are  given  off,  should  be  conducted  under  the  hood,  which 
should  have  an  efficient  draught.  This  is  necessary,  not  only  on 
account  of  the  comfort  and  health  of  the  analyst,  but  also  to  pre- 
vent contamination  of  the  laboratory  atmosphere.  Operations 
that  are  to  be  carried  out  under  the  hood  include:  the  evapora- 
tion of  silica  to  dryness  (p.  140)  the  evaporation  of  silica  with 
hydrofluoric  acid  (p.  145)  the  pyrosulphate  fusion  of  the  alumina 
precipitate  (p.  159),  the  decomposition  with  sulphuric  and  hydro- 
fluoric acids  for  the  determination  of  ferrous  oxide  (p.  187),  and 
the  digestions  with  sulphuric  and  hydrofluoric  acid  for  the  deter- 
mination of  manganous  oxide  (p.  221)  and  with  nitric  and  hydro- 
fluoric acids  for  that  of  phosphorus  pentoxide  (p.  217). 

5.  PRECIPITATION  1 

The  precipitant  should  be  capable  of  precipitating  quantita- 
tively the  substance  to  be  determined;  should  not  introduce  any 
substance  that  may  interfere  with  subsequent  precipitations  in 
the  same  liquid ;  and  any  excess  of  the  precipitant  should  be  readily 
removable  from  the  precipitate  by  washing. 

The  ideal  precipitate  should  be  insoluble  in  the  liquid  in  which 
it  is  formed;  should  be  wholly  precipitated,  and  as  quickly  as 

1Cf.  Fresenius,  p.  91;  Gooch,  p.  57;  Mellor,  p.  95;  Morse,  p.  198;  Ost- 
wald,  p.  75;  Stieglitz,  pp.  122-138,  145-155. 


88  OPERATIONS 

possible;  should  not  inclose  any  of  the  mother  liquor,  and  should 
not  adsorb  any  of  the  substances  present  in  the  solution;  should  be 
in  such  form  as  not  to  pass  through  the  filter,  and  be  readily  and 
completely  washed  free  from  impurities;  and,  finally,  should  be 
capable  of  being  changed  on  drying  or  ignition  to  a  definite  and 
stable  substance  of  known  and  invariable  composition. 

It  is  seldom,  if  ever,  that  we  can  attain  all  these  optimum 
desiderata,  and  we  must  rest  content  with  reducing  the  sources  of 
error  to  a  minimum.  The  following  rules  will  be  found  generally 
applicable,  but  precipitates  vary  so  much  in  character  that  some 
will  demand  special  modifications  of  treatment. 

The  solution  to  be  precipitated  should  seldom  be  highly  con- 
centrated, as  the  precipitate  will  then  be  more  apt  to  adsorb  sub- 
stances present  in  the  solution,  and  any  mother-liquor  retained 
by  it  will  constitute  a  relatively  great  impurity.  On  the  other 
hand,  it  should  not  be  unduly  dilute,  partly  because  no  precipitate 
is  absolutely  insoluble,  partly  because  more  time  will  be  needed  for 
complete  precipitation,  and  partly  because  it  will  add  to  the  volume 
of  the  liquid  to  be  filtered. 

The  precipitant  is  not  to  be  added  all  at  once,  but  slowly,  as 
this  will  tend  to  the  formation  of,  not  only  a  coarser,  and  more 
easily  washed,  precipitate,  but  one  less  contaminated  with  adsorbed 
substances. 

The  precipitant  should  not  be  added,  of  course,  in  such  a  way 
as  to  splash;  and  if  the  addition  is  likely  to  cause  bubbling  or 
effervescence  the  beaker  should  be  covered  with  a  watch-glass, 
and  the  liquid  added  through  a  funnel  with  bent  stem. 

The  solution  should  be  well  stirred  during  the  addition  of  the 
precipitant,  and  for  a  minute  or  so  after,  to  ensure  a  thorough 
mixture.  The  stirring  rod  should  be  so  long  as  to  be  handled  in 
the  subsequent  filtration  without  the  fingers  coming  in  contact 
with  the  precipitate  or  the  liquid  adhering  to  the  rod;  it  should 
project  about  5-7  cm.  above  the  final  bulk  of  liquid,  and,  of  course, 
extend  above  the  beaker  rim. 

A  decided,  but  not  extravagant,  excess  of  the  precipitant  must 
be  added  to  ensure  complete  precipitation.  This  is  because  of  the 
diminution  in  solubility  of  a  salt  in  solution  in  the  presence  of  an 
ion  common  with  it.1  The  addition  of  the  exactly  theoretical 

1  Cf.  J.  Walker,  Introduction  to  Physical  Chemistry,  pp.  329,  351,  1913. 


PRECIPITATION  89 

necessary  amount  of  precipitate  never  (or  almost  never)  produces 
complete  precipitation,  and  the  excess  may  be  defined  as  the  extra 
amount  needed  to  bring  this  about.1 

No  general  rule  can  be  laid  down  as  to  what  this  excess  should 
be;  but  the  beginner  should  avoid  adding  inordinate  amounts  of 
precipitant.  To  make  sure  that  one  has  added  sufficient,  until — 
and  in  some  cases  when — one  has  had  experience,  it  is  well,  after 
allowing  the  precipitate  to  settle,  to  add  a  few  drops  or  cubic 
centimeters  of  the  precipitant  and  observe  whether  there  is 
further  precipitation  after  a  short  time.  In  some  cases  consid- 
erable time  must  be  allowed  for  this,  and  then  the  test  may  be 
carried  out  in  the  first  few  cubic  centimeters  of  the  filtrate,  this 
being  returned  to  the  main  portion  if  a  precipitate  forms,  and  more 
precipitant  is  to  be  added  to  the  whole. 

Many  precipitates  are  so  fine-grained,  or  even  "  amorphous," 
when  first  formed  that  they  pass  through  the  filter.  This  is  pre- 
vented by  allowing  the  precipitate  to  stand  ("digest ")  for  some 
hours  in  the  solution  in  which  it  is  formed,  before  filtration.  In 
this  way  the  larger  crystals  present  grow  at  the  expense  of  the 
smaller,  and  the  whole  mass  of  precipitate  gradually  becomes  more 
coarsely  crystalline.2  Standing  in  a  warm  place,  as  at  the  back 
of  the  steam  bath,  will  facilitate  this.  Some  precipitates,  such  as 
magnesium  ammonium  phosphate,  form  very  slowly,  and,  with 
these,  standing  for  some  hours,  or  even  a  day,  is  necessary  for 
complete  precipitation. 

It  is  often  advantageous,  as  with  calcium  oxalate,  to  precipitate 
in  a  hot,  or  even  boiling,  solution,  as  this  usually  has  the  effect  of 
rendering  the  precipitate  more  coarsely  crystalline.  The  solution 
in  which  gelatinous  substances,  as  aluminum  and  ferric  hydroxide, 
are  precipitated,  should  always  be  hot. 

In  some  cases  it  is  well  to  add  to  the  solution,  before  precipi- 
tation, a  liquid  in  which  the  precipitate  is  but  slightly  soluble.3  This 
not  only  renders  the  precipitate  coarser-grained,  but  diminishes 
the  time  needed  for  standing  and  for  complete  precipitation.  For 
this  reason  it  is  well  to  add  alcohol  in  the  precipitation  of  calcium 
and  magnesium,  and  ammonia  water  as  well  in  that  of  the  latter. 

1  Cf.  Gooch,  p.  5,  Mellor,  p.  182,  note  6;  Ostwald,  p.  80. 

2  Cf.  Fresenius,  p.  92;  Gooch,  p.  58;  Mellor,  p.  96;  Ostwald,  p.  22. 

3  Cf.  Fresenius,  p.  92;  Gooch,  p.  58;  Ostwald,  p.  75. 


90  OPERATIONS 

Gelatinous  precipitates,  like  aluminum  and  ferric  hydroxides, 
are  not  only  very  difficult  to  wash  free  from  adsorbed  salts,  but 
tend  to  form  colloidal  solutions,  and  pass  through  the  filter.  This 
can  be  prevented  by  the  presence  of  electrolytes,  such  as  easily 
soluble  and  crystallizable  salts,  in  the  solution.1  Ammonium  salts, 
especially  the  chloride  and  nitrate,  are  generally  used  for  this  pur- 
pose, as  they  are  driven  off  on  ignition. 

Such  gelatinous  substances  should  always  be  precipitated  in 
hot  or  even  boiling  liquids.  After  precipitation  the  liquid  may  be 
brought  to  boiling,  as  this  effects  coagulation.  The  boiling  should 
not  be  long  continued,  as  this  tends  to  make  the  precipitate  slimy 
and  difficult  to  filter  and  wash.  Liquids  that  contain  gelatinous 
precipitates  are  very  liable  to  "  bump,"  which  may  easily  lead  to 
loss.  They  should,  on  this  account,  be  watched  during  the  heating. 

Because  of  the  impurities  present  in  almost  all  precipitates, 
and  especially  in  gelatinous  ones,  through  adsorption,2  co-precip- 
itation, or  inclusion  of  the  mother-liquor,  which  cannot  be  removed 
by  washing,  it  is  always  advisable,  if  not  necessary  for  good  work 
(when  the  substance  permits),  to  redissolve  the  precipitate  (after 
slight  washing),  and  to  reprecipitate  after  a  little  dilution.  If  the 
reprecipitation  is  effected  by  neutralization  of  the  acid  solution,  a 
small  amount  of  the  original  precipitant  is  previously  added  to 
ensure  the  presence  of  an  excess  of  a  common  ion. 

Before  filtration,  the  precipitate  is  allowed  to  stand  until  the 
solution  above  it  is  clear. 

Very  small  amounts  of  white  precipitates  may  escape  notice. 
They  may  be  detected  by  stirring  the  liquid,  when  the  precipitate 
collects  in  a  small  heap  at  the  center  of  the  bottom  of  the  beaker. 

6.  FILTRATION  AND  WASHING  3 

The  filtration  needed  in  the  analysis  of  rocks  is  of  two  kinds: 
simple  filtration,  in  which  a  filter  paper  is  used  and  where  the  fil- 
trate passes  through  by  its  own  weight  and  at  practically  atmos- 
pheric pressure;  suction  filtration,  in  which  either  filter  paper  or 

1  Cf.  Gooch,  p.  60;  Mellor,  p.  95;  Ostwald,  p.  24;  Stieglitz,  1,  pp.  125-138. 

2  Cf.  Gooch,  p.  61;  Mellor,  p.  96;  Ostwald,  pp.  18,  84. 

3Cf.  Fresenius,  pp.  94-109;  Gooch,  pp.  62-67;  Mellor.  pp.  90-106; 
Morse,  pp.  200-208;  Ostwald,  pp.  13-15;  Treadwell,  pp.  18-20. 


FILTRATION  AND  WASHING  9.1 

some  other  filtering  medium,  especially  asbestos,  is  used,  and  the 
process  takes  place  under  diminished  pressure,  so  that  the  filtrate 
is  "  sucked  "  through  the  filter.  Each  method  has  its  advantages, 
and  is  specially  suited  to  different  precipitates  and  conditions. 

Simple  Filtration. — This  method  is  that  most  generally  used, 
and  is  the  one  best  adapted  to  precipitates  that  can  be  ignited 
with  the  filter  paper  without  change  in  composition,  and  specially 
to  cases  where  the  filtrate  is  to  be  used  for  subsequent  precipita- 
tions. 

The  funnels  and  filter  papers  to  be  used  in  this  have  already 
been  described  (pp.  36,  44),  and  the  first  requisite  is  to  select  the 
appropriate  sizes  of  filter  and  funnel.  It  is  a  common  fault  of 
beginners  to  use  too  large  a  filter,  and  this  is,  as  Treadwell  says: 
"  One  of  the  inexcusable  analytical  errors." 

A  filter  paper  is  to  be  selected  that  is  as  small  as  possible,  so  as 
to  diminish  the  amount  of  washing  needed,  and  yet  that  will  con- 
tain all  the  precipitate  and  a  sufficient  amount  of  wash  water; 
on  the  other  hand,  it  must  not  be  too  small.  If  the  precipitate  is 
very  large  or  bulky  it  may  be  advisable  or  necessary  to  filter 
through  two  filters  simultaneously,  but  this  should  seldom  happen. 
The  size  will,  of  course,  differ  with  the  different  precipitates,  and 
appropriate  sizes  are  mentioned  later  in  the  course  of  the  descrip- 
tions of  the  different  methods. 

Two  principles  are,  however,  to  be  borne  in  mind.  The  first 
is  that  "  the  size  of  the  filter  used  should  be  regulated  entirely  by 
the  amount  of  the  precipitate  and  not  at  all  by  the  amount  of  the 
liquid  to  be  filtered."  (Treadwell,  p.  20.) 

The  second  is  that,  while  the  filter  should  be  as  small  as  possible, 
the  whole  precipitate  should  not  occupy  more  than  one-half,  and 
preferably  about  one-third  of  its  volume,  and  should  never  reach 
up  to  less  than  about  5  mm.  of  the  rim. 

The  funnel  to  be  used,  again,  should  not  be  too  large  for  the 
filter.  When  folded  and  put  in  place  the  edge  of  the  filter  should 
be  about  1,  at  most  not  more  than  2  cm.  below  the  edge  of  the 
funnel.  The  following  sizes  will  be  found  appropriate:1 

Filter    5J      7        9  11         12  J  cm. 

Funnel  3J    4-5    5-6J     6|-7£     7J-9  cm. 


Cf.  Mellor,  p.  90. 


92  OPERATIONS 

In  general,  the  9-cm.  filter  and  6J-cm.  funnel  are  the  most  used, 
with  the  11,  7,  and  5j-filters,  and  5  and  3i-funnels,  less  often 
needed. 

The  dry  filter  is  first  folded  exactly  in  half,  and  then  again  in 
half  (from  the  center-point  of  the  diameter) ,  so  as  to  form  a  quad- 
rant. The  folds  are  lightly  pressed  down  with  the  finger  tips, 
beginning  a  few  millimeters  from  the  tip  so  as  to  leave  this  un- 
pressed,  as  it  might  otherwise  break  or  leak.  The  dry  filter  is 
then  opened  out,  placed  in  the  funnel,  and  fitted  snugly  into  place, 
which  it  will  immediately  do  if  the  apical  angle  of  the  funnel  is 
60°.  If  this  is  not  exactly  60°,  the  paper  at  the  second  folding 
is  to  be  folded  from  the  center  of  the  diameter,  into  slightly  more 
than  a  quadrant;  and  is  then  opened  out  on  the  larger  or  smaller 
side  according  as  the  funnel  angle  is  greater  or  less  than  60 c.  A 
funnel  had  best  be  rejected  for  analytical  work  if  its  angle  is 
decidedly  greater  or  less  than  60°. 

Holding  the  paper  in  place  with  a  finger  tip,  the  filter  is  now  wet 
slightly  all  over  with  a  jet  of  water  from  the  wash-bottle.  Any 
excess  of  water  is  allowed  to  drain  through  the  stem,  and  is  not 
poured  out,  as  this  may  later  cause  "  creeping  "  of  fine  precip- 
itates up  the  glass.  With  a  finger  tip  the  filter  is  now  pressed 
gently  against  the  glass  all  over.  Care  should  be  taken  to  press 
out  any  air  bubbles,  and  to  press  down  the  creases  at  either  side 
made  by  the  folds,  along  their  length  and  especially  at  the  rim  of 
the  filter,  as  they  are  liable  to  form  air  channels  and  retard  filtra- 
tion or  lead  to  loss  of  precipitate  if  the  filter  is,  by  mischance,  over- 
filled. All  this  should  be  done  gently  and  without  any  rubbing 
motion,  or  the  tender,  moist  paper  may  be  torn.  If  this  happens 
the  torn  paper  is  to  be  rejected  and  a  new  one  used. 

The  funnel  is  then  placed  in  the  funnel  stand,  care  being 
taken  that  the  tip  of  the  stem  does  not  touch  the  wood  in  passing. 
Beneath  is  placed  a  beaker  of  the  size  suitable  to  hold  the  filtrate, 
as  will  be  suggested  in  the  descriptions  of  the  different  determina- 
tions. This  beaker  should  be  clean,  even  if  the  precipitate  is  to  be 
rejected,  and  it  is  best  to  rinse  it  out  first  with  a  little  water  from 
the  wash-bottle.  If  any  precipitate  passes  through  it  may  thus  be 
recovered  unchanged.  The  tip  of  the  funnel  stem  should  be  close 
to,  but  not  touching,  the  side  of  the  beaker,  and  be  within  about 
5  cm.  of  the  bottom,  to  prevent  spattering.  As  the  filtrate  accu- 


FILTRATION  AND  WASHING  93 

mulates  in  the  beaker  the  arm  of  the  funnel  stand  is  to  be  raised 
from  time  to  time,  so  as  to  keep  the  tip  about  the  same  height  above 
the  surface  of  the  liquid. 

The  beaker  with  the  liquid  and  precipitate  to  be  filtered,  which 
has  been  standing  until  the  liquid  above  is  perfectly  clear,  is  held 
in  the  right  hand  1  above  the  funnel,  the  stirring  rod  (held  in  the 
left  hand)  rested  vertically  against  the  lip  of  the  beaker,  projecting 
about  5  cm.  below  this,  and  the  beaker  tilted  gently  until  the  liquid 
flows  slowly  down  the  rod  2  and  into  the  filter.  The  stream  is  to  be 
directed  against  the  side,  not  the  tip,  of  the  filter,  and  this  should 
be  filled  only  to  within  about  2-3  mm.  of  the  rim,  so  that  the  edge 
is  kept  quite  free  from  precipitate.  The  liquid  should  never  be 
allowed  to  overflow  the  filter.3 

If  the  filter  has  been  properly  fitted  and  the  funnel  and  suction 
tube  are  clean,  the  tube  will  fill  with  a  column  of  liquid  that  causes 
a  gentle  suction.  Generally,  after  the  column  is  established  the 
liquid  will  flow  through  in  a  steady  stream  for  the  first  few  minutes. 
During  this  time  the  beaker  and  rod  are  to  be  kept  steadily  in 
position  above  the  filter,  the  tilt  of  the  beaker  being  gradually 
increased  so  as  to  maintain  the  level  of  the  liquid  in  the  filter. 
There  must  be  no  "  slopping,"  and  as  little  of  the  precipitate  as 
possible  should  enter  the  filter. 

After  a  time  the  filtrate  begins  to  pass  more  slowly  and  to  issue 
from  the  stem  in  drops,  because  of  the  clogging  of  the  paper  with  a 
little  of  the  precipitate.  The  beaker  may  be  then,  from  time  to 
time  (to  rest  the  hand) ,  replaced  on  the  bench,  and  the  stirring-rod 
placed  in  it,  but  without  disturbing  or  stirring  up  the  precipitate 
or  resting  against  the  lip.  As  has  been  already  said,  the  rod  should 
be  long  enough  to  permit  handling  without  touching  the  adherent 
precipitate  with  the  fingers.  The  liquid  should  be  thus  poured 
into  the  filter  from  time  to  time,  as  it  empties,  not  allowing  the 
filter  to  empty  so  far  as  to  break  the  column  in  the  suction  tube. 

1  Here  and  elsewhere  it  is  assumed  that  the  operator  is  right-handed. 

2  It  is  inadvisable  to  pour  the  liquid  directly  from  the  beaker  into  the  filter 
without  the  aid  of  a  rod,  as  it  is  apt  to  drip  or  splash. 

3  However  clear  the  upper  liquid  may  appear,  or  however  large  its  volume, 
it  must  never  be  "  decanted  "  directly  into  the  other  beaker,  but  should  all  be 
passed  through  the  filter.     Particles  of  precipitate  float  on  the  surface  or  in  the 
mass  of  liquid  and  may  escape  notice,  but  they  would  be  lost  unless  caught 
on  the  filter. 


94  OPERATIONS 

If  an  air-channel  develops  beneath  one  of  the  creases,  allowing 
bubbles  to  pass  into  the  tube,  the  open  upper  ends  are  to  be  gently 
pressed  down  and  closed  with  the  tip  of  the  stirring-rod,  without 
breaking  the  paper. 

As  more  and  more  of  the  liquid  passes  into  the  filter  the  last 
portion  left  in  the  beaker  becomes  quite  thick  with  the  precipitate.1 
If  the  precipitate  is  to  be  redissolved,  as  will  almost  always  be  the 
case,  most  of  the  precipitate  should  not  be  allowed  to  pass  into  the 
filter.  The  sides  of  the  beaker  are  to  be  washed  down  with  a  little 
water,  directed  from  the  tip  of  the  wash-bottle  all  around,  from  the 
upper  level  of  the  adherent  precipitate  down.  After  standing  a 
few  minutes  and  allowing  the  precipitate  to  settle,  the  liquid  is 
poured  into  the  filter  as  before,  as  little  as  possible  of  the  pre- 
cipitate going  with  it.  This  washing  is  repeated  only  two  or  three 
times.  The  beaker  with  the  precipitate  is  then  substituted  for 
that  with  the  filtrate,  the  exchange  being  made  so  that  no  drops 
are  lost. 

A  sufficient  amount  of  the  appropriate  solvent  (generally  a 
dilute  acid)  is  prepared  in  a  small  100  c.c.  beaker,  and  the  filter 
filled  nearly  to  the  top  with  this.  As  it  dissolves  the  precipitate 
in  the  filter  and  passes  through  the  suction  tube  2  it  is  allowed  to 
drop  against  the  side  of  the  beaker  below,  above  the  upper  line  of 
adherent  precipitate.  The  beaker  is  held  somewhat  slanting  and 
close  to  the  tip  of  the  stem  so  that  the  drops  do  not  bounce  off. 
As  the  solvent  drops,  the  beaker  is  turned  around  until  all  the  pre- 
cipitate covering  its  interior  is  acted  on  and  dissolved. 

When  the  filter  is  empty  it  is  again  filled  with  the  solvent,  and, 
if  necessary,  the  solution  of  adherent  precipitate  completed,  while 
the  stirring-rod  is  cleaned  of  precipitate  by  some  drops  of  solvent 
running  down  it.  After  several  repetitions,  by  which  time  all 
the  precipitate  in  the  filter  should  be  dissolved,  and  during  which 
the  beaker  below  is  placed  upright  on  the  base  of  the  stand  and  the 
funnel  is  lowered  to  prevent  splashing,  not  more  than  about  50  c.c. 
of  solvent  should  have  been  used. 

1  The  description  now  applies  to  ordinary,  crystalline  precipitates.     Gel- 
atinous ones  need  slightly  different  treatment,  as  will  be  described  under  silica 
(p.  140),  and  alumina  (p.  152). 

2  A  turbidity  appearing  in  the  suction  tube  is  but  momentary  and  need 
cause  no  concern.     It  is  due  to  neutralization  of  the  acid  by  the  ammonia 
remaining  in  the  tube  causing  reprecipitation. 


FILTRATION  AND  WASHING  95 

The  filter  is  then  washed  four  or  five  times,  the  liquid  passing 
of  course,  into  the  beaker  below,  and  the  sides  of  the  beaker  and 
the  stirring-rod  are  washed  down.  This  should  all  be  done  so 
that  not  more  than  100  or  150  c.c.  of  liquid  and  washings  containing 
the  dissolved  precipitate  are  in  the  original  beaker. 

The  liquid  is  then  warmed,  a  few  drops  of  the  precipitant 
added  and  reprecipitation  brought  about  by  the  addition  of 
the  proper  reagent.  During  this  operation  the  contents  of 
the  beaker  are  to  be  stirred  constantly.  The  beaker  (covered) 
is  set  aside  for  some  time  to  ensure  settling  and  complete 
precipitation. 

If  the  precipitate  is  not  to  be  reprecipitated,  the  whole  loose 
precipitate,  after  standing,  is  brought  into  the  filter  with  the  liquid. 
The  sides  of  the  beaker,  from  above  the  line  of  adherent  precip- 
itate, are  washed  down  once  or  twice  and  after  each  washing  the 
contents  of  the  beaker  are  poured  into  the  filter,  which  is  each 
time  allowed  to  empty.  In  what  follows  the  filter  is  allowed  to 
drain  after  each  addition. 

The  beaker  is  then  taken  in  the  left  hand,  held  tilted  slightly 
down  (mouth  to  the  right)  and  lip  down  and  above  the  filter. 
The  rod  is  placed  across  the  beaker,  resting  in  the  lip  and  the  tip 
projecting  a  few  centimeters,  the  upper  part  held  firmly  in  place 
with  the  left  forefinger.  The  interior  is  then  washed  with  a  jet  of 
water,  commencing  on  the  now  upper  side,  and  sweeping  the  pre- 
cipitate down.  Small  portions  of  water  are  to  be  used  at  a  time, 
and  all  the  loose  precipitate  is  to  be  washed  into  the  filter. 

The  beaker  being  set  down,  its  walls  and  the  rod  are  well  wet 
with  water,  and  the  precipitate  adherent  to  them  is  rubbed  loose 
with  a  "  policeman  "  or  rubber-tipped  rod.  This  should  be  done 
systematically  and  thoroughly. 

The  rod  is  attended  to  first.  Held  above  the  beaker,  the 
portion  with  adherent  precipitate  is  rubbed  along  its  whole  length, 
the  rod  being  turned  so  that  the  whole  surface  is  treated,  the  tip 
not  being  neglected.  It  is  then  washed  off  with  a  little  jet  of 
water  into  the  beaker,  and  laid  across  the  funnel,  after  examina- 
tion to  see  that  it  is  free  from  precipitate. 

The  interior  of  the  beaker  is  then  rubbed  clean,  also  syste- 
matically. This  may  best  be  done  by  rubbing  a  sector  of  a  few 
centimeters  wide  at  a  time,  rubbing  sidewise  from  top  to  bottom; 


96  OPERATIONS 

then  another  adjacent  sector,  and  so  on.  The  rubber  is  occasion- 
ally wet  with  the  liquid  in  the  bottom  of  the  beaker.  The  bottom 
is  finally  rubbed  clean,  best  beginning  with  complete  circular 
sweeps  around  the  bend,  and  gradually  working  in  to  the  center. 
The  rubber  is  washed  into  the  beaker. 

All  the  now  loose  precipitate  is  then  washed  into  the  filter,  as 
just  described.  The  interior  of  the  beaker  is  examined  in  a  good 
light,  and  any  remaining  patches  of  adherent  precipitate  are 
rubbed  off  and  washed  into  the  filter,  until  the  beaker  and  rod  are 
entirely  clean  and  all  the  precipitate  has  been  brought  into  the 
filter. 

The  whole  amount  of  precipitate  should  not  more  than  half 
fill  the  filter,  and  should  not  reach  to  above  5  mm.  of  its  edge. 
It  is  best  that  the  mass  of  precipitate  does  not  have  a  flat  surface 
but  be  rather  thicker  and  lower  toward  the  apex  and  thinning 
out  of  the  sides.1 

Washing  of  Precipitates.2 — The  precipitate  has  now  to  be 
washed,  and  in  doing  this  one  has  to  steer  between  the  Scylla  of 
not  washing  it  free  from  all  contaminating  salts,  and  the  Charybdis 
of  over-washing  and  thereby  dissolving  some  of  it.  It  is,  there- 
fore most  desirable  not  to  wash  beyond  the  point  at  which  the 
impurities  in  the  wash  liquid  and  adherent  to  the  precipitate  are 
just  entirely  removed  or  are  reduced  to  unweighable  (and  there- 
fore negligible)  amount.  This  will  also  have  the  advantage  of 
keeping  the  bulk  of  filtrate  at  a  minimum.  The  theory  of  the 
matter  is  discussed  in  sufficient  detail  by  the  authors  cited  above 
so  it  need  not  be  gone  into  here. 

The  general  principle  to  which  the  discussion  leads,  and  which 
should  be  borne  in  mind  and  applied  in  all  washings,  is  that,  given 
a  certain  amount  of  liquid,  the  washing  is  more  efficient,  and, 
therefore,  less  wash  liquid  is  needed,  if  it  is  done  with  many  small 
portions  rather  than  with  a  few  large  portions.  In  the  washing, 
therefore,  the  precipitate  should  be  little  more  than  covered  with 
water  and  allowed  to  drain  each  time. 

As  to  the  total  amount  of  wash  liquid  (say  water)  that  should 
be  used  no  generally  applicable  rule  can  be  given,  as  precipitates 

1  Cf.  Treadwell,  p.  20,  Fig.  4. 

2  Fresenius,  1,  p.  98;  Gooch,  p.  62;  Mellor,  pp.  95-98;  Morse,  pp.  205-208; 

,...1,1      -r-  ~      IK     OO  •     TVn^n/Jnmll      v^»-»      1Q     OA 


*  .Fresenius,  i,  p.  y»;  uoocn,  p.  oz;  iviei 
Ostwald,  pp.  15-22;  Treadwell,  pp.  18-20 


FILTRATION  AND  WASHING  97 

vary  much  in  their  characters.  Theoretically,  washing,  that  is 
mixing  up  and  rather  more  than  covering  the  precipitate,  four  to 
six  times  (the  filter  being  emptied  each  time),  should  remove  sol- 
uble salts  to  such  an  extent  that  the  residue  is  negligible.  This, 
however,  is  seldom  actually  true,  and  most  precipitates  need  to  be 
washed  (in  the  sense  above)  many  more  times,  up  to  ten  or  twenty, 
or  more,  before  the  process  is  complete. 

It  may  be  remarked  that  the  tendency  of  beginners  is  both  to 
add  too  large  portions  at  a  time,  and  to  overwash,  though  there  are 
exceptions  where  impatience  leads  to  underwashing. 

In  washing  precipitates,  several  practical  precautions  are  to  be 
observed.  The  jet  of  water  is  first  directed  against  the  edge  of  the 
filter,  and  carried  two  or  three  times  around  the  upper  zone  that  is 
free  from  precipitate.  This  will  clear  this  portion  of  soluble  salts. 

The  first  impact  of  the  jet  should  never  be  directed  against  the 
precipitate,  as  otherwise  some  of  the  precipitate  is  liable  to  be 
thrown  out  and  lost.  It  should,  therefore,  be  aimed  first  at  a 
clean  space  of  the  filter  and  from  this  gently  directed  against  the 
edge  of  the  precipitate. 

After  washing  the  rim  clean,  the  precipitate  is  gradually  worked 
downward  with  the  jet,  letting  the  filter  empty  after  it  becomes 
about  half  full.  When  all  of  it  has  been  collected  at  the  bottom  of 
the  filter,  the  mass  of  precipitate  should  be  well  mixed  with  water 
several  times,  great  care  being  taken  that  none  of  it  is  thrown  out 
by  too  violent  an  application  of  the  jet,  and  that  not  too  much 
water  is  used.  Each  time*  some  of  the  precipitate  will  be  again 
spread  above  the  main  portion,  but  at  the  end  of  the  washing  the 
precipitate  should  be  all  collected  at  the  bottom  of  the  filter,  with 
the  upper  part  of  the  sides  clean.  A  little  practice  will  enable 
one  to  do  this  easily. 

It  is  to  be  remembered  that  the  filter  must  not  be  allowed  to 
overflow. 

After  (not  before)  the  precipitate  has  been  mixed  with  water 
three  or  four  times,  it  will  be  well  to  test  the  filtrate  for  an  impurity, 
usually  chloride  or  sulphate,  that  will  indicate  whether  the  wash- 
ing is  complete  or  not.  For  this,  the  tip  of  the  suction  tube  is 
washed  off  with  a  small  jet  of  water,  as  the  previous  portions  of 
filtrate  are  apt  to  creep  up  the  outside  and  thus  show  impurity 
when  the  actual  liquid  in  the  filter  is  free  from  it.  A  little  more 


98  OPERATIONS 

water  is  added  to  the  precipitate,  the  funnel  is  raised  after  the 
column  of  liquid  in  the 'tube  has  been  driven  out,  and  a  few  drops 
of  the  uncontaminated  filtrate  are  caught  in  a  small,  clean  wash- 
glass.  These  are  then  tested  with  the  appropriate  reagent,  silver 
nitrate  fcr  chlorides,  after  the  addition  of  a  drop  of  nitric  acid  and 
barium  chloride  for  sulphates.  By  this  time  the  drops  should 
contain  so  negligible  an  amount  of  dissolved  salts  that  they  are  to 
be  rejected,  even  if  they  show  a  reaction.  This  testing  is  to  be 
repeated  after  each  four  or  five  washings,  until  there  is  no  reaction. 
The  precipitate  is  now  ready  for  drying  and  ignition  (p.  101). 

Suction  Filtration. — This  method  has  the  advantage  of  greatly 
shortening  the  time  necessary  for  filtration,  but  is  not  well  adapted 
to  cases  where  the  filtrate  is  to  be  used  later.  The  Gooch  modifi- 
cation, however,  is  especially  useful,  indeed  now  almost  indispen- 
sable, when  the  precipitate  undergoes  change  on  ignition  with 
filter  paper,  or  if  it  cannot  be  ignited  and  must  be  dried  at  a  low 
temperature. 

The  simplest  form,  in  which  the  filter  is  of  paper  and  supported 
in  the  funnel  by  a  perforated  platinum  cone,  to  prevent  rupture 
under  pressure,  is  so  seldom  used  in  the  analysis  of  rocks  that  little 
need  be  said  of  it  here. 

A  funnel  with  ordinary  stem  is  used,  this  fitting  tightly  a  single- 
perforated  rubber  stopper  that  closes  a  filtering  flask  with  stout 
walls.  If  it  is  desired  to  use  the  filtrate  later,  this  flask  must  be 
well  cleaned,  but  it  is  better  to  use  Witt's  filtering  apparatus,  with 
the  stem  of  the  funnel  projecting  into  an  Erlenmeyer  flask. 

The  perforated  platinum  cone,  best  seamless,  should  fit  the 
funnel  accurately.  Especial  care  should  be  bestowed  in  fitting  the 
paper  closely  against  both  the  metal  and  the  glass,  which  may  be 
done  by  folding  the  paper  the  second  time  into  very  slightly 
(l°-2°)  more  than  a  quadrant.  If  it  is  not  properly  fitted  it  will 
almost  surely  be  torn  at  the  junction. 

The  suction  should  be  applied  gently  and  increased  gradually, 
and  the  final  pressure  should  not  be  too  great;  only  experience 
can  teach  what  this  should  be.  The  filtration  and  washing  are  con- 
ducted as  in  the  simple  method,  except  that  the  funnel  may  be 
emptied  more  frequently,  though  in  the  washing  the  precipitate 
should  be  well  stirred  with  water  to  guard  against  the  formation  of 
channels. 


FILTRATION  AND  WASHING  99 

Gooch  Crucible. — In  the  Gooch  method  the  filter  consists  of  a 
layer  of  asbestos  supported  on  the  perforated  bottom  of  a  crucible, 
the  filtration  being  aided  by  rather  strong  suction.  The  crucible 
may  be  of  either  platinum  or  porcelain,  but  the  former  is  preferable, 
as  it  is  heated  and  cooled  much  more  quickly.  The  appropriate 
size  has  been  mentioned  on  p.  31,  and  the  preparation  of  the 
asbestos  has  been  described  on  p.  49.  As  an  example  of  the 
usual  practice,  the  filtration  of  ammonium  magnesium  phosphate 
may  be  taken;  that  of  potassium  platinichloride  demands  some 
slight  modifications  in  detail,  which  will  be  noted  in  their  proper 
place  (p.  205). 

The  carbon  filter  tube,  inserted  in  a  closely  fitting  rubber  stop- 
per, has  a  band  of  soft  rubber  tubing  about  3  cm.  long  slipped  over 
the  wide  end,  which  it  should  fit  closely  for  about  half  its  length; 
and  the  other  half,  which  is  bent  over  horizontally,  is  cut  with 
scissors,  until  only  a  narrow  ring  of  rubber  surrounds  the  interior 
of  the  rim.1 

The  stopper  is  fitted  tightly  into  the  filtering  flask,  and  this  is 
connected  with  the  suction.  A  little  of  the  well-shaken  asbestos 
suspension  is  poured  in — about  2  or  3  c.c.,  but  experience  must 
teach  one  the  right  amount.  The  felt  when  dry  should  not  be  more 
than  about  1  or  2  mm.  thick,  and  may  weigh  about  5-10  centigrams. 
The  suction  is  turned  on  gently  and  the  crucible  emptied.  It  is 
filled  with  water  five  or  six  times,  the  jet  being  directed  gently 
against  the  side  so  as  not  to  tear  the  felt.  It  is  thus  washed  until 
no  asbestos  fibers  come  through,  50  to  100  c.c.  of  water  being 
ample,2  and  is  finally  sucked  dry  under  somewhat  increased 
pressure. 

It  is  sometimes  advisable  to  use  a  thin,  circular,  perforated 
disc,  which  is  placed  on  a  thin  layer  of  wet  asbestos  first  applied 
and  sucked,  and  the  disc  covered  with  an  upper  layer  of  asbestos. 
Otherwise  the  filter  is  prepared  as  above.  Though  the  disc  retards 
the  filtration  somewhat,  it  protects  the  felt  and  decreases  the  lia- 
bility of  this  being  torn.  If  due  care  is  used,  however,  I  do  not 
find  the  use  of  a  disc  to  be  necessary. 

1  This  is  preferable  to  the  arrangement  suggested  by  Morse,  p.  202,  and 
Mellor,  p.  104. 

2 1  do  not  think  that  the  large  volumes  recommended  by  Mellor  (p.  105) 
are  either  necessary  or  advisable. 


100  OPERATIONS 

The  crucible  is  heated  over  a  low  flame,  the  bottom  cap  being 
left  off,  and  the  flame  moved  about  by  hand.  In  this  way  the 
felt  is  well  dried  without  loosening  or  blistering,  as  the  steam 
generated  from  its  lower  side  will  escape  through  the  perforations. 
When  quite  dry,  as  is  indicated  by  the  whiteness  of  the  asbestos, 
the  bottom  cap  is  put  on,  and  the  crucible  is  covered  and  ignited 
at  a  bright-red  heat  for  a  short  time,  to  drive  off  all  traces  of  water.1 
It  is  then  cooled  in  the  desiccator  and  weighed. 

The  Gooch  crucible,  with  the  bottom  cap  and  cover  removed, 
is  placed  in  position  in  the  carbon  filter,  care  being  taken  when 
inserting  it  in  the  rubber  mouth,  that  the  latter  does  not  come  in 
contact  with  the  bottom  of  the  crucible  and  rub  off  any  small 
pieces  of  asbestos  which  may  project  beyond  the  perforations. 
In  order  to  prevent  loosening  of  the  felt  by  the  upward  pressure 
of  the  air  in  the  flask,  the  suction  should  be  turned  on  before 
inserting  the  crucible  in  the  filter  tube. 

The  suction  should  be  gentle  and  at  the  same  time  effective. 
The  liquid  is  poured  slowly  into  the  crucible,  the  stream 
from  the  stirring-rod  striking  the  side  and  not  directly 
on  the  felt.  Otherwise  the  latter  is  liable  to  be  torn  and 
some  of  the  perforations  laid  bare,  possibly  allowing  some  of 
the  fine  precipitate  or  asbestos  to  pass  through.  The  whole  of 
the  liquid  is  thus  filtered,  with  considerable  of  the  precipitate 
entering  the  crucible,  so  as  to  protect  the  felt.  The  beaker  is  then 
rinsed  with  a  stream  of  (in  this  case)  very  dilute  ammonia  water 
several  times,  the  bulk  of  the  precipitate  going  into  the  Gooch 
crucible.  The  adhering  precipitate  is  loosened  from  the  sides  and 
bottom  of  the  beaker  and  from  the  stirring-rod  by  means  of  a  rub- 
ber-tipped rod,  as  already  described,  and  the  last  traces  of  it 
brought  into  the  filter  by  gentle  streams  of  the  dilute  ammonia 
from  the  wash-bottle. 

The  precipitate  on  the  felt  is  well  washed  with  the  same  fluid, 
the  stream  being  directed  against  the  side  of  the  crucible,  not  on 
the  felt.  The  crucible  is  allowed  to  empty  before  each  addition,  of 
which  about  half  a  dozen  will  be  sufficient  in  most  cases.  If 
desired,  the  washing  can  be  tested  by  stopping  the  suction,  remov- 
ing the  stopper  of  the  Erlenmeyer  flask,  and  letting  a  few  drops 

1  If  there  is  no  bottom  cap,  or  the  crucible  be  of  porcelain,  it  is  placed 
within  another,  larger,  platinum  crucible  for  ignition. 


DRYING  AND  IGNITIOtf  101 

fall  on  a  watch-glass.  These  are  acidified  with  a  drop  or  two  of 
nitric  acid  and  tested  with  silver  nitrate.  It  will  be  well  to  do 
this  the  first  few  times,  till  one  learns  by  experience  when  the  pre- 
cipitate is  thoroughly  washed. 

When  the  washing  is  complete,  the  suction  is  continued  for  a 
short  time  in  order  to  partially  dry  the  precipitate,  the  connection 
is  shut  off,  and  the  stopper  is  cautiously  loosened,  so  as  to  prevent 
regurgitation  of  any  liquid  that  may  remain  in  the  tube,  up 
against  the  felt.  The  bottom  cap,  if  there  be  one,  is  put  on,  and 
the  crucible  is  ready  for  ignition  (p.  105). 

7.  DRYING  AND  IGNITION  l 

When  the  precipitate  has  been  filtered  and  washed,  it  has  to  be 
dried  and  brought  into  a  condition  of  stable  and  definite  composi- 
tion for  weighing.  The  method  to  be  adopted  for  this  purpose 
depends  on  whether  or  not  the  precipitate  is  changed  by  heating 
with  filter  paper  or  on  ignition  at  a  high  temperature. 

Drying. — When  it  is  a  matter  of  drying  at  a  low  temperature 
such  as  with  the  rock  powder  for  hygroscopic  water,  or  potassium 
platinichloride,  the  substance,  contained  in  an  ordinary  platinum 
or  a  Gooch  crucible,  according  to  circumstance,  is  dried  in  the  hot- 
air  oven. 

Weighed  filter  papers  should  never  be  used,  because  of  the 
hygroscopicity  of  paper.  The  Gooch  crucible  has  entirely  sup- 
planted, or  at  least  should  entirely  supplant  them,  so  that,  as 
Mellor  (p.  102)  says,  their  use  will  soon  be  obsolete. 

The  crucible  containing  the  precipitate  is  covered  with  a  small 
(7  cm.)  filter  paper  and  placed  in  the  oven.  The  paper  prevents 
contamination  by  particles  falling  from  the  oven  top,  and  at  the 
same  time  allows  the  steam  to  escape.  The  regular  crucible  cover 
is  laid  aside  in  a  clean  place.  The  door  of  the  oven  is  closed,  the 
burner  underneath  is  lit,  and  the  temperature  is  brought  gradually 
to  110°,  130°,  or  whatever  may  be  required.  It  is  held  at  this  for 
the  requisite  time  by  adjustment  of  the  burner  from  time  to  time, 
if  there  is  no  regulator  attached.  A  little  practice  soon  teaches 
one  the  right  height  of  flame. 

1Fresenius,  1,  pp.  112-120;  Mellor,  pp.  168,  183,  213,  219;  Treadwell, 
pp.  21-29. 


102  OPERATIONS 

The  crucible  is  then  placed  in  the  desiccator,  its  cover  is  put  in 
place,  and  it  is  allowed  to  cool  for  weighing. 

If  the  precipitate  is  collected  in  a  paper  filter  it  should  always 
be  ignited  from  a  moist  condition.  If  heating  and  ignition  with 
paper  or  carbon  changes  the  precipitate,  the  Gooch  crucible  should 
be  used. 

The  practice,  much  recommended  l  and  frequently  used,  of 
drying  the  precipitate  and  filter,  then  removing  the  greater. part 
of  the  precipitate,  burning  the  filter  paper  separately,  and  igniting 
the  precipitate  and  ash,  is  one  that,  like  weighed  paper  filters, 
should  never  be  used.  The  manipulation  is  very  apt,  if  not  almost 
certain,  to  lead  to  the  loss  of  some  of  the  dry  powder,  and  this 
antiquated  method  should  be  superseded  by  the  use  of  the  Gooch 
crucible. 

The  ignition  of  precipitates  in  a  moist  paper  filter  is  a  simple 
procedure,  and  is  by  far  the  one  most  frequently  used  in  the  analysis 
of  rocks. 

The  free  edges  of  the  moist  filter  containing  the  precipitate  in 
the  funnel  are  loosened  and  then  folded  down  all  around  over  the 
precipitate.  This  is  done  with  the  platinum  spatula,  the  end  of 
which  should  not  be  soiled  by  the  precipitate.  In  this  way  the 
mass  of  moist  precipitate  is  completely  inclosed  in  paper. 

The  funnel  being  held  in  the  left  hand,  the  little  package  is 
gently  loosened  and  lifted  by  the  platinum  spatula  and  is  placed 
in  the  bottom  of  a  previously  weighed  platinum  crucible.  This 
should,  if  possible,  be  not  more  than  half  filled  by  the  package. 
This  is  laid  with  the  threefold  side  uppermost,  and  is  pressed 
snugly  down,  but  without  breaking  the  paper,  and  leaving  a  chan- 
nel on  either  side  for  the  exit  of  steam  from  beneath.  The  used 
tip  of  the  spatula  is  cleaned  on  a  small  scrap  of  filter  paper,  say  one- 
quarter  of  a  7-cm.  filter.  With  the  same  piece  of  paper,  folded 
once  to  cover  the  possible  trace  of  precipitate  from  the  spatula 
the  funnel  is  rubbed  all  around  at  the  line  of  the  filter  edge  to 
remove  any  precipitate  that  may  possibly  have  crept  up.  The 
paper,  folded  up,  is  placed  in  the  crucible  alongside  the  main 
mass. 

In  this  way  the  precipitate  may  be  dried  in  the  crucible  without 
danger  of  loss  from  whirling  up  of  powder  or  from  handling  of  the 
1  Fresenius,  1,  p.  510;  Treadwell,  p.  21. 


DRYING  AND  IGNITION  103 

crackly  and  somewhat  brittle  dried  paper  that  is  involved  in  the 
earlier  method.     Incineration  of  the  filter  will  be  complete. 

The  crucible,  with  the  cover  in  place,  is  supported  vertically 
in  a  triangle  and  heated  above  a  rather  low  flame  of  a  Bunsen 
burner.  The  appropriate  conditions  will  vary  somewhat,  but  for 
this  stage  of  the  drying  the  flame  should  be  5-7  cm.  high  and  the 
bottom  of  the  crucible  10  or  better  15  cm.  above  its  tip.  The 
object  is  to  dry  the  moist  mass  of  precipitate  without  boiling  of 
the  pasty  mass  or  spattering  against  the  sides  of  the  crucible.  One 
should  be  patient  and  heat  with  great  caution  to  prevent  this. 

When  the  mass  is  dry,  and  there  are  no  more  drops  of  water 
on  the  under  side  of  the  cover,  the  crucible  is  gently  and  gradually 
lowered,  a  little  at  a  time.  When  doing  this  the  ring  on  which  the 
triangle  rests  should  be  steadied  with  the  tongs,  while  the  clamp  is 
loosened  and  the  ring  lowered. 

The  paper  is  carbonized  thus  very  gradually,  which  is  an  advan- 
tage; as  paper  carbonized  at  a  low  temperature  is  more  easily 
incinerated  than  if  carbonized  rapidly  and  at  a  high  temperature. 
The  escaping  vapors  must  never  be  allowed  to  ignite,  as  this  will 
very  easily  cause  loss  of  the  powder  through  the  slight  explosion 
at  ignition  and  the  currents  that  are  set  up.  Until  the  paper  is 
wholly  carbonized,  as  is  shown  by  the  pure  black  color,  the  bottom 
of  the  crucible  is  not  brought  close  enough  to  the  flame  to  become 
red  hot. 

Ignition. — When  the  paper  is  entirely  carbonized  the  crucible 
is  lowered  and  the  flame  slightly  raised,  so  that  the  bottom  of  the 
crucible  is  at  a  low  red  heat.  The  cover  is  slipped  a  little  to  one 
side  so  as  to  allow  a  little  air  to  enter,  the  crucible  being  still  ver- 
tical, and  not  on  its  side  at  this  stage,  as  is  often  recommended. 
The  red  heat  at  the  bottom  is  maintained  until  the  paper  is  wholly 
burnt  away,  the  flame  being  somewhat  raised  toward  the  last,  but 
never  enough  to  envelop  the  crucible.1 

If  the  carbon  of  the  filter  paper  or  that  which  penetrates  the 
precipitate  burns  slowly,  its  combustion  may  be  hastened  by 

1  The  flame  should  not  roar,  as  it  so  frequently  does  in  unpracticed  hands, 
but  the  gas  and  air  should  be  so  adjusted  that  it  burns  quietly,  and  without  any 
luminous  portion.  The  adjustment  may  have  to  be  altered  with  the  raising 
and  lowering  of  the  flame.  A  roaring  Bunsen  burner  flame,  though  it  sounds 
like  a  blast,  is  no  more  effective  than  a  quiet  one,  and  is  a  noisy  nuisance  to 
one's  neighbors. 


104  OPERATIONS 

removing  the  flame  and  allowing  the  aii*  to  penetrate  the  cold 
carbon.  On  reheating,  combustion  will  usually  be  rapid.  The 
under  side  of  the  crucible  cover  should  always  be  examined  to  see 
if  it  is  free  from  carbon.  If  some  still  adheres  after  the  carbon 
of  the  paper  is  consumed  it  is  burnt  off  by  holding  the  cover  in 
the  platinum-tipped  tongs  convex  side  up  above  the  flame  for  a 
few  minutes. 

In  general  the  crucible  is  then  ignited  at  a  bright  red  heat 
(about  900°  C.),  with  the  crucible  vertical,  the  cover  very  slightly 
to  one  side,  and  the  flame  not  enveloping  the  crucible.  This  igni- 
tion may  last  for  thirty  minutes,  but  for  many  precipitates  fifteen 
to  twenty  minutes  will  suffice.  The  flame  is  then  turned  off,  the 
crucible  allowed  to  cool  below  redness,  and,  while  still  warm 
placed  in  the  desiccator  to  cool  before  weighing.  Until  one  has 
had  experience  as  to  the  time  of  ignition,  it  is  well,  after  weighing, 
to  reheat  for  ten  minutes,  cool  and  again  weigh,  until  there  is  no 
loss  of  weight. 

With  some  precipitates,  especially  where  an  oxidizing  atmos- 
phere is  necessary,  as  with  ferric  oxide,  the  crucible  is  laid  on  its 
side  on  the  triangle,  not  quite  horizontal,  the  mouth  toward  one 
of  the  ends.  The  cover  is  leaned  against  it  with  the  handle  to  one 
side,  almost,  but  not  quite,  closing  it.  If  the  end  of  the  triangle 
is  not  twisted,  a  few  notches  may  be  filed  along  one  of  the  ends  to 
keep  the  cover  from  slipping.  The  flame  is  applied  .only  to  the 
lower  end  of  the  crucible,  not  near  the  mouth. 

This  position  of  the  crucible  is  much  recommended,  but,  in  my 
opinion,  is  not  well  adapted  to  most  precipitates,  especially  those 
(such  as  silica  or  calcium  oxide)  which  are  in  the  form  of  light 
powders,  as  the  danger  of  loss  from  draughts  is  very  great.  The 
crucible  and  cover  should  never  be  placed  in  the  quite  irrational 
position  sometimes  figured.1  The  flame  must  always  play  on  the 
base  of  the  crucible,  never  around  its  mouth. 

The  much-used  method  of  drying  the  precipitate  and  carbon- 
izing the  paper  by  heating  the  handle  of  the  cover  resting  on  the 
vertical  crucible  is  to  be  discouraged.  Besides  being  unnecessarily 
tedious,  there  is  great  probability  of  the  flame  gases  entering  the 
crucible  and  causing  the  presence  of  a  reducing  atmosphere. 

In  igniting  a  precipitate  in  a  Gooch  crucible  the  procedure  is 
1  Gooch,  p.  76;  Treadwell,  p.  29. 


TITRATION  105 

much  the  same.  If  the  crucible  has  a  bottom  cap,  this  is  put  on 
and  the  covered  crucible  heated  above  a  low  flame  as  described 
above  until  the  felt  and  precipitate  are  dry,  after  which  it  is  ignited 
at  a  bright  red  heat.  If  there  is  no  cap,  the  Gooch  crucible  is 
placed  in  a  larger  platinum  crucible  and  heated  and  ignited  in 
the  same  way. 

Some  precipitates,  for  example,  silica  and  alumina,  require 
blasting.  This  is  done  after  the  crucible  and  contents  have  been 
heated  at  bright  redness  for  fifteen  minutes.  In  blasting,  the  cru- 
cible should  be  vertical,  the  cover  very  slightly  slipped  to  one  side 
(about  1  mm.).  The  flame  of  the  blast  should  not  be  large  and 
should  be  directed  only  against  the  bottom  and  lower  third  of  the 
crucible. 

For  many  purposes  the  Meker  burner,  which  gives  a  hotter 
flame  than  the  Bunsen,  will  take  the  place  of  a  blast.  Though  it 
may  take  slightly  longer  to  dehydrate  the  substance,  there  is  less 
liability  of  loss  of  weight  in  the  platinum  through  vaporization  if 
the  blast  be  not  used. 

If  a  good  grade  of  filter-paper  be  used,  such  as  those  recom- 
mended elsewhere,  the  weight  of  filter  ash  may  be  neglected  in 
the  calculations,  as  it  will  fall  within  the  other  limits  of  error. 
The  only  general  exception  would  be  in  the  case  of  the  precipitate 
by  ammonia,  for  alumina,  etc.,  when  three  or  more  11-cm.  filters 
are  ignited.  The  combined  weight  of  their  ash  may  not  be  negli- 
gible in  accurate  work,  and  should  be  deducted  from  the  weight 
of  the  ignited  precipitate. 

8.  TITRATION  1 

In  the  ordinary  course  of  rock  analysis,  volumetric  operations 
are  made  use  of  only  in  the  determination  of  the  iron  oxides,  and 
in  the  colorimetric  methods  for  titanium  manganese,  and  chro- 
mium. They  can  be  used  as  well  for  the  determination  of  other 
constituents,  but  this  is  not  recommended  in  rock  analysis,  where 
gravimetric  methods  are  preferred  for  the  great  majority  of 

1  The  various  text-books  frequently  cited  throughout  these  pages,  espe- 
cially Fresenius,  Gooch,  Mellor,  Morse,  and  Treadwell,  describe  the  various 
procedures  of  volumetric  analysis  in  more  or  less  detail ;  they  should  be  con- 
sulted by  the  student  for  the  principles  and  information  that  is  not  given  here. 


106  OPERATIONS 

determinations.  No  full  discussion  of  volumetric  methods,  or 
their  principles,  will  be  attempted  here,  but  a  few  practical  sug- 
gestions will  be  made  that  may  be  of  use  to  the  student. 

Volume-Burette. — Two  types  of  burette  are  in  general  use,  the 
volume  and  the  weight-burette. 

If  the  volume-burette  most  often  seen  is  of  common  Mohr's 
form,  a  capacity  of  50  c.c.,  graduated  to  tenths  of  1  c.c.,  is  of  an 
appropriate  size.  The  stopcock  should  be  of  glass,1  and  it  is  best 
set  at  an  angle  of  90°.  These  burettes  should  be  calibrated,  or  of 
the  so-called  "  precision  "  grade,  for  good  work,  and  they  should 
be  consistent  with  the  measuring  flasks  used.2 

The  burette  should  be  perfectly  dry  before  the  standard  solu- 
tion is  introduced,  and  it  is  well  to  rinse  it  out  with  a  few  cubic 
centimeters  of  the  solution  before  filling.  The  standard  solution 
should  be  taken  from  the  stock  bottle  with  a  pipette,  not  poured 
out,  as  this  last  will  disturb  its  titer.  Before  taking  out  any 
standard  solution  the  bottle  containing  it  is  to  be  gently  shaken, 
so  as  to  wash  down  any  drops  of  water  that  may  have  been  depos- 
ited by  evaporation  on  the  upper  walls.  The  solution  is  to  be 
introduced  into  the  burette  above  the  zero  mark,  and  allowed  to 
run  out  until  it  is  level  with  this.  The  tip  of  the  burette  should  be 
filled  in  doing  this. 

The  precautions  to  be  observed  in  reading  the  position  of  the 
meniscus,  so  as  to  avoid  the  effects  of  parallax,  are  described  by 
several  of  the  authorities  mentioned  above.  As  we  have,  in  rock 
analysis,  to  deal  only  with  liquids  that  are  transparent  and  light- 
colored,3  the  lower  edge  of  the  meniscus  is  to  be  used.  For 
reading  the  position  of  this,  I  prefer  a  general  illumination  giving 
the  effect  shown  by  Fresenius.4  The  eye  must  be  level  with  the 
meniscus,  but  such  devices  as  floats,  striped  cards,  or  a  vertical 
dark  stripe  at  the  back,  are  generally  neither  necessary  nor 

1  Permanganate  solutions  must  never  come  into  contact  with  rubber,  and 
glass  stopcocks  are  much  the  best  for  all  purposes. 

2  In  the  colorimetric  methods  for  which  they  are  used  slight  discrepancies, 
say  one-tenth  of  a  cubic  centimeter,  are  of  no  very  serious  consequence,  as  the 
amounts  of  the  constituents  determined  are  small  and  the  consequent  errors 
negligible. 

3  The  permanganate  solution  of  proper  strength  for  rock  analysis  is  so  light 
in  color  that  the  bottom  of  the  meniscus  can  be  easily  seen  through  it. 

4  Fresenius,  1,  p.  46,  Fig.  19. 


TITRATION  107 

advisable.  One  can  use  a  strip  of  white  paper,  with  a  straight 
edge  uppermost,  bent  around  the  burette  and  with  the  ends  of 
the  straight  edge  meeting.  This  is  moved  up  or  down  until  the 
front  and  rear  portions  of  the  straight  edge  are  in  line  with  the 
lowest  point  of  the  meniscus. 

In  reading  a  volume-burette  a  few  minutes  must  be  allowed  for 
the  drainage  of  the  liquid  adhering  to  the  walls  above  the  meniscus, 
or  the  reading  will  be  too  high.  The  burette  may  be  gently  tapped 
to  facilitate  this.  For  all  but  the  most  accurate  work  there  is  no 
need  for  corrections  for  temperature.1 

Weight-burette.2 — For  accurate  work  the  weight-burette  has 
some  great  advantages  over  the  ordinary  form,  which  make  it 
preferable  for  use  in  the  determination  of  the  iron  oxides.  These 
advantages  are:3  no  calibration,  is  required;  no  correction  for 
temperature  changes  is  necessary;  it  is  not  necessary  to  wait  for 
drainage;  reading  the  position  of  a  meniscus  is  not  involved;  the 
solution  can  be  weighed  readily  to  0.01  gram,  whereas  measure- 
ment to  0.01  c.c.  is  very  uncertain. 

The  burette  is  provided  with  a  hollow,  ground-glass  stopper,  in 
which  is  a  small  hole,  there  being  a  similar  hole  in  the  neck.  During 
weighing,  the  stopper  is  turned  so  that  the  two  holes  do  not  coin- 
cide, thus  closing  the  burette;  before  titration  the  two  holes  are 
made  to  coincide,  so  that  air  may  enter  and  the  solution  escape.  The 
glass  protecting  cap  should  be  placed  on  the  tip  during  weighing. 

Before  use,  the  burette  should  be  dry,  and  some  cubic  centi- 
meters of  the  solution  (permanganate)  to  be  used  is  shaken  in  the 
burette  and  allowed  to  run  out;  this  serves  to  clean  it  and  at  the 
same  time  saturates  the  air  in  the  burette  with  the  vapor  of  the 
solution.  The  burette  of  appropriate  size  (100  cc.  or  50  c.c.)  is 
filled  nearly,  but  not  quite  to  the  stopper,4  this  is  put  into  place 
with  the  two  holes  not  coinciding,  and  the  protective  cap  is 
placed  on  the  tip,  great  care  being  taken  that  the  plug  of  the 
stopcock  is  not  loosened.  The  burette  is  then  wiped  dry  with 

1  A  table  for  these  is  given  by  Treadwell,  2,  pp.  534,  535. 

2  M.  Ripper,  Chem.  Zeit.,  p.  793,  1892. 

3  E.  W.  Washburn,  Jour.  Am.  Chem.  Soc.,  30,  p;  40,  1908;  R.  S.  McBride, 
Bull.  Bur.  Stand.,  8,  p.  617,  1912. 

4  The  graduations  on  the  side  serve  as  a  guide  to  the  rough  measure  of  the 
amount  of  liquid  used  during  the  titration;  they  are  not  to  be  used  for  measur- 
ing the  exact  volume. 


108  OPERATIONS 

a  clean  towel,  and  is  weighed.  In  carrying  the  burette  to  the  bal- 
ance case  it  is  well  to  hold  the  burette  by  the  neck  with  the  fore- 
finger and  thumb  of  one  hand,  and  with  a  finger  of  the  other  press- 
ing lightly  against  the  cap,  so  that  this  may  not  slip  off  and  be 
broken.  The  burette  may  be  handled  with  dry  fingers.  In 
weighing,  it  is  suspended  by  a  loop  of  wire  attached  to  it  from  the 
hook  above  the  left-hand  pan.  After  weighing  it  is  placed  in  the 
clamp  with  rubber-covered  jaws,  which  are  gently  screwed  up  so 
as  to  hold  the  burette  in  place,  but  without  danger  of  crushing  it. 

The  Operation. — To  allow  the  liquid  to  escape,  in  working  with 
either  form  of  burette,  the  cock  is  best  managed  with  the  left 
hand,  the  right  being  used  to  rotate  or  stir  the  liquid  in  the  iron 
titration.  The  sides  of  the  plug  and  of  the  socket  should  be 
straight  and  strictly  conical  and  well-fitting,  so  that  they  do  not 
jam  on  turning.  In  turning  the  cock,  care  should  be  taken,  on 
the  one  hand  not  to  bear  down  so  heavily  as  to  cause  the  plug  to 
stick  in  the  socket,  and  on  the  other  not  to  raise  the  plug  so  as  to 
let  liquid  escape  between  the  plug  and  socket.  The  tip  should  not 
be  so  high  above  the  liquid  below  as  to  cause  splashing  of  drops. 

The  liquid  must  be  added  cautiously,  and  after  each  addition, 
the  flask  is  rotated  or  the  liquid  stirred.  Toward  the  end,  the 
addition  must  be  drop  by  drop,  great  caution  being  needed  not 
to  overrun  the  end-point.  When  the  total  amount  is  known 
approximately,  then  the  first  additions  of  the  standard  solution 
may  be  relatively  large  but  the  end-point  must  be  approached 
with  the  same  degree  of  caution. 

The  end-point,  as  in  iron  titrations,  is  partly  a  matter  of  judg- 
ment as  to  what  shade  of  color  is  to  be  taken  as  representing  it. 
In  iron  titrations  the  first  pink  blush,  that  remains  permanent 
after  a  minute's  stirring  or  rotation,  is  to  be  taken;  but  this  should 
correspond  with  that  which  has  been  selected  in  the  standardiza- 
tion of  the  permanganate  solution.  A  single  drop  should  be  suf- 
ficient to  cause  the  change,  though  in  the  titration  for  ferrous  oxide 
the  decision  as  to  the  end-point  is  complicated  by  the  presence  of 
hydrofluoric  acid,  which  renders  the  color  evanescent,  as  we  shall 
see. 

More  details  will  be  found  in  the  descriptions  of  the  vari- 
ous methods  in  which  a  burette  is  used,  those  for  ferric  and  ferrous 
oxides,  and  the  colorimetric  methods  for  titanium  and  manganese. 


PART  V 

METHODS 
1.  GENERAL  COURSE  OF  ANALYSIS 

BEFORE  beginning  the  detailed  descriptions  of  the  methods  for 
determining  l  the  various  constituents,  it  will  be  advisable  to  state 
concisely  what  the  course  of  analysis  is,  in  what  separate  portions 
the  different  constituents  are  determined,  and  the  plan  of  separa- 
tion, in  order  to  obtain  a  general  survey  of  the  analysis,  so  that 
the  details  may  be  considered  later  with  greater  intelligence  and 
knowledge  of  their  relations  to  the  whole  analysis.  In  this  sum- 
mary, if  there  are  several  alternative  methods  which  are  described 
subsequently,  only  that  one  will  be  mentioned  which  especially 
recommends  itself  for  the  use  of  students,  and  which,  in  general, 
I  have  adopted  for  my  own  work. 

a.  Silica,  alumina,  total  iron  oxides,  titanium  dioxide,  lime, 
strontia,  and  magnesia,  are  determined  in  a  portion  of  1  gram, 
which  is  usually  called  the  "  main  "  portion.  The  powder  is  fused 
with  five  times  its  weight  of  sodium  carbonate,  and  the  cold  cake  is 
dissolved  in  hydrochloric  acid  and  the  solution  evaporated  to  dry- 
ness,  thus  rendering  the  silica  insoluble.  The  silica  is  filtered  off 
and  in  the  filtrate,  alumina,  ferric  oxide,  titanium  dioxide  and  phos- 
phorus pentoxide  are  precipitated  by  ammonia  water,  with  or 
without  the  addition  of  ammonium  persulphate.  After  filtration 
the  precipitate  is  dissolved  in  nitric  acid  and  reprecipitated  by 
ammonia,  and  this  is  repeated  if  there  is  much  magnesia  present. 
The  precipitate  is  ignited  and  weighed,  and  then  brought  into 
solution  by  fusion  with  potassium  pyrosulphate.  The  melt  is 

1The  phrase  "to  determine"  is  more  appropriate  than  "to  estimate"; 
the  former  is  denned  as  "to  ascertain  definitely,"  and  the  latter  as  "  to  form 
a  judgment  regarding  the  value,  etc.,  of."  The  former  is  definite,  the  latter  is 
approximate.  Compare  Mellor,  p.  672. 

109 


110  METHODS 

dissolved  in  water,  the  ferric  oxide  is  reduced  by  hydrogen  sulphide, 
the  excess  of  this  boiled  off,  and  the  total  iron  oxide  is  determined 
by  titration  with  potassium  permanganate.  Titanium  dioxide  is 
determined  in  the  same  liquid  by  the  colorimetric  method,  which 
consists  in  comparing  the  intensity  of  color  of  a  known  volume  of 
the  liquid  after  oxidation  by  hydrogen  peroxide,  with  that  of  a 
standard  solution  of  titanium  colored  in  the  same  way. 

The  filtrate  from  the  ammonia  precipitate  is  precipitated 
with  ammonium  oxalate,  the  precipitate  of  calcium  oxalate 
dissolved  and  reprecipitated,  and  the  lime  determined  as  such 
by  ignition  of  the  oxalate. 

Strontia  may  be  determined  in  the  weighed  lime,  obtained 
as  above,  by  solution  in  nitric  acid,  evaporation  to  dry  ness, 
solution  of  the  calcium  nitrate  by  a  mixture  of  ether  and  absolute 
alcohol,  solution  of  the  strontium  nitrate  in  water  and  precipitation 
of  the  strontium  as  sulphate  after  addition  of  alcohol. 

In  the  filtrate  from  the  calcium  oxalate  the  magnesia  is  deter- 
mined by  precipitation  as  ammonium-magnesium  phosphate, 
which,  after  solution  and  reprecipitation,  is  ignited.  The 
magnesia  is  weighed  as  pyrophosphate.  The  filtrate  from  this 
last  operation  is  rejected. 

b.  Ferrous  oxide  is  determined  in  a  portion  of  powder  of  half 
a  gram  by  solution  in  a  boiling  mixture  of  hydrofluoric  and  sul- 
phuric acids,  the  operation  being  conducted  in  a  well-closed 
platinum  crucible.  The  contents  of  the  crucible  are  transferred 
to  water  and  titrated  with  potassium  permanganate. 

(c)  Alkalies  are  determined  in  a  portion  of  half  a  gram  of  spe- 
cially ground  powder  which  is  effected  by  the  Lawrence  Smith 
method.     The  powder  is  intimately  mixed  with  ammonium  chlo- 
ride and  calcium  carbonate,  and  heated  under  the  proper  condi- 
tions.    After  thorough  leaching,  the  filtrate  is  precipitated  with 
ammonium  carbonate,  and  the  filtrate  from  this  is  evaporated 
to  dryness.     The  ammonium  chloride  is  driven  off  by  cautious 
heating,  and  the  chlorides  of  sodium  and  potassium  are  weighed. 
The  potassium  is  separated  by  chloroplatinic  acid,  and  is  weighed 
as  platinichloride,  the  soda  being  determined  from  the  weight 
of  the  mixed  chlorides  by  difference. 

(d)  Combined  water  is  determined  by  PenfiekTs  method  in  a 
portion  of  J  to  1  gram.     The  powder  is  ignited  in  a  dry  glass  tube 


GENERAL  COURSE  OF  ANALYSIS  111 

sealed  at  one  end,  and  the  water  driven  to  the  cool  portion  of  the 
tube;  the  end  containing  the  powder  is  drawn  off,  and  the  water 
weighed  in  the  remaining  portion.  The  amount  of  hygroscopic 
water  is  deducted. 

(e)  Hygroscopic  water  is  determined  in  a  portion  of  about  1 
gram  by  heating  at  a  temperature  of  110°.  This  portion  is  to  be 
used  afterward  for  the  determination  of  other  constituents,  as 
P2O5,  MnO,  or  S,  Zr02,  and  BaO. 

(/)  Phosphorus  pentoxide  is  determined  by  digestion  of  a 
portion  of  about  1  gram  with  nitric  and  hydrofluoric  acids,  removal 
of  silica  by  evaporation,  and  subsequent  precipitation  as  ammo- 
nium phosphomolybdate.  This  precipitate  is  dissolved  in  am- 
monia water,  the  phosphorus  is  thrown  down  by  magnesia 
mixture  as  ammonium  magnesium  phosphate  and  weighed  as 
magnesium  pyrophosphate. 

(g)  Manganous  oxide  is  determined  in  a  portion  of  1  gram,  the 
rock  powder  being  broken  up  by  heating  with  sulphuric  and  hydro- 
fluoric acids,  the  latter  being  driven  off.  To  the  filtrate  a  solution 
of  silver  nitrate  and  some  ammonium  persulphate  are  added  and 
the  liquid  heated,  the  manganous  salt  being  oxidized  to  per- 
manganate. The  manganese  in  this  solution  is  then  determined 
colorimetrically,  by  comparison  with  a  standard  solution  of 
manganous  sulphate  similarly  treated. 

(h)  Total  sulphur,  zirconia,  the  rare  earths,  and  baryta  may 
be  determined  in  a  portion  of  1  gram.  The  rock  powder  is  fused 
with  sodium  carbonate,  and  the  melt  leached  with  water.  After 
acidification  of  the  filtrate  with  hydrochloric  acid,  the  sulphur  is 
precipitated  and  weighed  as  barium  sulphate.  The  zirconia  is 
dissolved  out  of  the  residue  insoluble  in  water  by  very  dilute  sul- 
phuric acid,  and,  after  addition  of  hydrogen  peroxide,  is  thrown 
down  and  weighed  as  basic  phosphate  by  the  addition  of  sodium 
phosphate.  In  the  filtrate  from  this  precipitate  the  rare  earths 
may  be  determined.  The  barium  remains  as  sulphate  after  solution 
of  the  zirconia.  It  is  brought  into  solution  by  fusion  with  sodium 
carbonate,  which  converts  it  into  carbonate,  the  melt  leached 
with  hot  water,  and  the  residue  dissolved  in  hydrochloric 
acid.  It  is  precipitated  as  sulphate,  in  which  form  it  is 
weighed. 

(i)  Sulphur  trioxide  is  determined  in  a  portion  of  about  1  gram 


112  \        METHODS 

by  digestion  with  dilute  hydrochloric  acid  and  precipitation 
as  barium  sulphate. 

(j)  For  chlorine  a  portion  of  1  gram  is  digested  with  chlorine- 
free  nitric  acid,  and  the  chlorine  precipitated  in  the  filtrate  by 
silver  nitrate. 

(k)  Fluorine  is  determined  in  a  portion  of  2  grams  by  fusion 
with  sodium  carbonate,  leaching  with  water,  and  precipitation  of 
the  filtrate  with  ammonium  carbonate,  the  filtrate  from  which  is 
precipitated  with  an  ammoniacal  solution  of  zinc  oxide.  In  the 
filtrate  from  this  a  mixture  of  calcium  carbonate  and  fluoride  is 
precipitated  by  calcium  chloride,  and  the  calcium  carbonate  dis- 
solved out  by  acetic  acid,  leaving  the  calcium  fluoride,  in  which 
form  the  fluorine  is  weighed. 

(I)  A  portion  of  from  2  to  5  grams  is  used  for  the  determination 
of  carbon  dioxide.  The  rock  powder  is  decomposed  by  hydro- 
chloric' acid  in  a  small  flask,  and  the  carbon  dioxide  is  absorbed 
in  a  weighed  U-tube  containing  soda-lime,  precautions  being 
taken  to  keep  the  apparatus  full  of  a  current  of  air  free  from  car- 
bon dioxide,  and  to  properly  dry  and  purify  the  gas  given  off  from 
the  rock. 

(m)  For  chromium  sesquioxide  a  gram  of  rock  powder  will  suf- 
fice, though  2  grams  are  preferable.  After  fusion  with  alkali 
carbonate  and  a  little  potassium  nitrate,  and  subsequent  leaching 
with  water,  the  chromium  is  determined  as  chromate  in  the  filtrate 
by  a  colorimetric  comparison  of  a  known  volume  of  the  solution 
with  a  standard  solution  of  potassium  chromate.  If  necessary, 
the  solution  is  concentrated  by  evaporation  before  making  the 
comparison. 

In  regard  to  the  weight  of  the  portions  which  it  is  recom- 
mended to  take  for  the  various  determinations,  it  should  be 
borne  in  mind  that  they  are  intended  for  the  great  majority  of 
rocks,  but  that  in  exceptional  cases  they  are  to  be  departed  from 
according  to  the  judgment  of  the  analyst.  For  instance,  in  the 
analysis  of  iron  ores,  if  a  gram  be  taken  for  the  main  portion  the 
bulk  of  the  voluminous  precipitate  of  ferric  hydroxide  will  be  so 
great  that  it  cannot  all  be  brought  on  one  filter,  and  possibly  not 
on  two.  Of  such  rocks,  therefore,  only  half  a  gram  of  powder 
need  be  taken,  even  though  extra  care  must  be  paid  to  the  deter- 
mination of  other  constituents.  On  the  other  hand,  for  the 


TIME   NEEDED  FOR  AN  ANALYSIS  113 

determination  of  alkalies  in  peridotites  and  other  rocks  in  which 
their  amount  is  extremely  small,  a  gram  or  two  of  powder  should 
be  taken,  instead  of  the  half  gram  which  is  usually  sufficient. 

The  criticism  is  sometimes  made  that  portions  of  1  gram  or  of 
\  gram  are  so  small  that  they  are  inadequate  to  yield  on  analysis 
a  just  idea  of  the  actual  proportions  of  the  several  constituents  in  a 
mass  of  rock.  It  is,  of  course,  generally  true  that  the  larger  the 
amount  of  substance,  within  limits,  that  is  taken  for  analysis  the 
greater  will  be  the  accuracy;  that  is,  the  more  negligible  will  be 
the  errors  incident  to  the  various  operations  in  comparison  with 
the  mass  taken.1 

Considerations  of  practicality,  however,  such  as  the  amount  of 
material  available,  the  practically  manageable  sizes  of  utensils, 
and,  above  all,  the  time  involved  in  the  different  operations,  set 
limits  on  the  amount  of  the  portion  to  be  taken.  Morse  points 
out  also  the  consideration  that  the  use  of  large  quantities  permits 
of  (and  therefore  tends  to  encourage)  careless  methods  of  work 
without  seemingly  impairing  the  accuracy  of  the  results. 

We  must  compromise  between  the  theoretical  advantage  of 
greater,  and  the  practical  advantages  of  smaller,  weight  of  sub- 
stance taken.  Long  experience  of  analysts  has  clearly  taught  us 
what  are  the  optimum  amounts  to  be  taken  in  particular  cases, 
and  the  student  should  adhere  to  these  whenever  possible;  it 
being  assumed  that  the  specimen  of  rock  powder  has  been  properly 
sampled  and  so  the  whole,  and  consequently  a  portion  of  it,  is 
representative  of  the  rock  mass. 

2.  TIME  NEEDED  FOR  AN  ANALYSIS 

In  answer  to  the  question  "  How  long  does  it  take  to  make  a 
complete  rock  analysis?"  Hillebrand 2  states  that,  given  ample 
and  adequate  laboratory  facilities  and  apparatus,  it  is  possible  for 
an  experienced  and  quick  worker  to  complete  an  analysis  of  a 
series  every  three  days,  after  the  first  is  finished,  barring  delays. 
While  this  is  possible,  the  analyst  is  not  generally  dealing  with 
a  series  of  analyses,  but  is  more  concerned  about  the  time  needed 
for  the  completion  of  a  single  one. 

1  Cf.  Morse,  p.  214;  Fresenius,  1,  p.  71. 

2  Hillebrand,  p.  30. 


114  METHODS 

The  time  necessary  for  performing  the  separate  parts  of  the 
various  analytical  operations  are  more  or  less  fixed  at  minima  by 
the  conditions  and  circumstances  of  each.  The  times,  however, 
may  often  be  very  largely  controlled  by  the  skill  and  judgment  of 
the  analyst.  Thus,  although  the  calcium  oxalate  precipitate 
must  stand  for  a  given  number  of  hours,  the  careful  and  experi- 
enced analyst  will  take  much  less  time  than  the  beginner  in  its 
filtration,  and  will  use  much  less  water  in  washing  it,  and  so  lessen 
the  time  needed  for  the  magnesia  determination.  Again,  the 
skillful  analyst  will  carry  out  two  or  three  filtrations  simulta- 
neously, while  other  operations  are  proceeding  automatically; 
the  beginner  will  find  his  time  fully  occupied  with  one  filtration, 
and  will  probably  have  forgotten  to  utilize  the  time  for  automatic 
parts  of  other  operations. 

The  beginner  in  analytical  work  has  a  strong  tendency  to  use 
too  large  vessels  and  filters  and  to  overwash  precipitates;  both 
of  which  lead  to  unnecessarily  large  amounts  of  filtrates.  He 
also  is  very  apt  to  ignite  precipitates  for  an  inordinate  length  of 
time.  All  such  practices,  which  are  mostly  due  to  conscientious 
excess  of  care,  not  only  increase  the  time  needed  for  analysis,  and 
in  the  aggregate  greatly  so,  but  make  for  lessening  rather  than  for 
increasing  the  accuracy  of  the  work. 

The  beginner  is  also  very  apt  to  leave  a  slow-running  filtration 
"  for  a  minute,"  forget  it  in  the  interest  of  some  other  process, 
and  thus  lose  time  or  get  into  difficulties,  for  example,  with  gela- 
tinous precipitates. 

Besides  the  general  fundamentals  of  care,  cleanliness,  and  con- 
scientiousness, there  are  two  short  maxims  that  it  will  be  well  for 
the  beginner  to  bear  constantly  in  mind  and  conform  to,  if  he  wishes 
to  complete  an  analysis  in  a  reasonably  short  time.  These  are: 
"  Be  always  on  the  job  "  and  "  Keep  the  sizes  and  volumes  small." 

The  analyst  should  not  be  content  to  wait  for  each  partial 
operation  to  be  terminated  before  beginning  another,  but  should 
avail  himself  of  the  opportunities  which  present  themselves  for 
carrying  on  simultaneously  as  many  separate  operations  as  it  is 
possible  to  do  with  success.  The  ability  to  do  this  naturally  grows 
with  experience  in  the  purely  mechanical  execution,  and  also  with 
judgment  as  to  the  best  way  of  economizing  time.  It  is  not  recom- 
mended that  the  novice  should  attempt  very  much  in  this  way, 


TIME  NEEDED  FOR  AN  ANALYSIS  115 

and  he  will  probably  find  that  two  or  three  operations  at  once  are 
all  that  he  can  cope  with  successfully  at  the  start.  But  he  should 
constantly  bear  in  mind  the  manifold  possibilities  in  this  direction, 
and,  with  growing  experience,  avail  himself  of  the  various  oppor- 
tunities that  present  themselves. 

With  some  practice,  the  number  of  different  operations,  both 
active  and  passive,  which  may  be  conducted  simultaneously  or 
nearly  so,  may  easily  reach  six  or  more.  Thus  during  the  filtra- 
tion of  the  first  precipitate  of  ammonium  magnesium  phosphate, 
the  following  operations  may  be  carried  out :  The  evaporation  of 
the  solution  of  the  alkali  chlorides,  the  expulsion  of  EkS,  from 
the  reduced  iron  solution,  and  the  ignition  of  the  precipitate  of 
calcium  oxalate.  As  a  matter  of  fact,  the  precipitates  by  which 
phosphorous  pentoxide,  sulphur,  baryta  and  zirconia  are  deter- 
mined may  be  ready  for  filtration  about  the  same  time  as  the 
ammonium  magnesium  phosphate  and  the  filtration  of  about  two 
of  these  precipitates  may  be  carried  out  with  the  latter.  Any  such 
combination  implies,  of  course,  a  sufficiently  liberal  supply  of 
apparatus  so  as  not  to  be  kept  waiting  for  lack  of  the  necessary 
utensils,  and  it  also  assumes  that  the  analyst  may  devote  several 
hours  continuously  at  a  time  to  the  analysis. 

To  come  to  concrete  figures,1  it  is  easily  possible  to  finish  an 
analysis  involving  the  determination  of  eighteen  or  twenty  con- 
stituents in  five  days,  not  necessarily  consecutive,  of  eight  or  nine 
hours  each  without  interruptions,  and  even  in  less  time.  Such  an 
analysis  can  surely  be  made  in  six  days  without  any  special  effort 
at  economizing  time.  Indeed,  a  comparatively  simple  analysis, 
in  which  a  dozen  constituents  are  to  be  determined,  may  be  com- 
pleted readily  in  four  days  without  any  sacrifice  of  accuracy,  but 
this  last  is  possible  only  in  the  hands  of  a  quick  and  experienced 
worker  with  proper  facilities. 

In  the  present  section  some  suggestions  are  made  of  the  possi- 
bilities in  the  way  of  economizing  the  time  of  analysis.2  They  are 
not  intended  to  be  final,  but  will  serve  merely  as  guides  in  laying 
out  the  plan  of  analytical  work,  and  are  subject  to  modification 
to  suit  the  exigencies  of  each  particular  case.  In  connection 

1  Hillebrand,  p.  30. 

2  These  suggestions  are  based  on  an  analysis  made  especially  for  this  pur- 
pose, the  results  of  which  are  given  on  pp.  243-246. 


116  ".     METHODS 

with  them  some  estimates  are  given  of  the  time  which  is  needed 
for  the  several  operations  and  determinations.  These,  again, 
must  be  regarded  as  only  rough  approximations,  which  will  vary 
with  different  laboratory  facilities  and  with  the  skill  and  experience 
of  the  operator.  They  will  have  to  be  extended  somewhat  when 
the  analysis  is  conducted  by  a  novice. 

Assuming  that  we  start  Monday  morning  at  nine  o'clock,  with 
about  50  grams  of  rock  chips,  these  can  be  reduced  to  powder 
ready  for  analysis  in  less  than  an  hour.  Weighing  out  the  crucible 
and  the  main  portion  (p.  129)  will  take  about  fifteen  minutes. 
The  main  fusion  with  sodium  carbonate  (p.  131)  is  then  begun 
at  about  ten  o'clock,  the  time  needed  for  the  fusion  and  cooling 
being  about  one  hour.  During  this  time  another  separate  portion 
can  be  weighed  out,  dried  for  half  an  hour  at  110°  for  the  hydro- 
scopic  water,  and  the  portion  mixed  with  sulphuric  and  hydro- 
fluoric acids  and  started  evaporating  for  the  determination  of 
manganous  oxide  (p.  219). 

When  the  carbonate  cake  is  cold  it  is  transferred  to  the  plat- 
inum basin.  About  one  hour  will  be  consumed  in  bringing 
this  cake  into  solution.  The  evaporation  of  this  liquid  to  dry  ness 
to  render  the  silica  insoluble  will  take  until  nearly  three  o'clock. 
During  this  time  the  determination  of  alkalies  can  be  begun  (p.  191). 
The  special  grinding  and  weighing  out  of  the  powder,  and  mixing 
with  calcium  carbonate  and  ammonium  chloride,  will  take  one-half 
to  three-quarters  of  an  hour,  and  the  subsequent  ignition  another 
three-quarters  of  an  hour,  during  which  a  portion  can  be  weighed 
out  for  phosphorus  pentoxide  and  its  evaporation  with  nitric  and 
hydrofluoric  acids  be  begun  (p.  216). 

One  should  have  time  toward  the  end  of  the  afternoon  to  wash 
the  silica  and  begin  the  evaporation  of  the  filtrate  for  the  recovery 
of  the  extra  trace  of  silica.  This  second  evaporation  may  be  con- 
tinued until  the  close  of  the  day  (it  will  take  about  one  hour  and  a 
half),  or  it  may  be  continued  on  the  steam  bath  over  night.  If 
that  is  not  available  it  can  be  finished  the  first  thing  next  morning. 
It  will  be  seen  that  the  first  day  is  mainly  one  of  decompositions 
and  preparation. 

On  Tuesday  morning  the  silica  is  first  attended  to.  The  wash- 
ing of  the  recovered  silica,  and  the  ignition  and  burning  off  of  the 
small  filter  will  not  take  more  than  half  an  hour,  after  which  the 


TIME   NEEDED   FOR  AN  ANALYSIS  117 

heating  and  ignition  of  the  silica  will  begin,  which  will  take  an 
hour  and  a  half  to  two  hours. 

During  the  ignition  of  the  silica,  the  precipitation  and  filtra- 
tion of  the  ammonia  precipitate  (p.  146)  can  be  begun.  If 
three  precipitations  are  necessary  this  will  take  nearly  three  hours, 
and  will  demand  almost  constant  attention.  During  this  time, 
however,  the  ignited  silica  can  be  weighed  and  its  evaporation  with 
hydrofluoric  acid  started,  the  evaporation  taking  nearly  an  hour. 
This  can  be  finished  by  the  time  the  final  ammonia  precipitate  is 
ready  to  put  in  the  crucible  with  the  residue  from  the  silica  and  its 
heating  is  begun.  It  may  be  possible  to  get  this  alumina  precip- 
itate dried  and  ignited  by  the  end  of  the  day,  if  it  is  not  large. 

When  the  filtrations  from  the  alumina  precipitate  are  finished 
they  are  brought  to  a  boil  and  the  lime  is  precipitated  with  ammo- 
nium oxalate  (p.  177).  It  is  better,  and  involves  no  real  loss  of 
time,  to  let  the  beakers  with  the  calcium  oxalate  precipitates 
stand  over  night. 

Time  should  be  found  this  afternoon  (Tuesday)  to  leach  the 
cake  from  the  alkali  fusion,  which  has  been  soaking  in  a  little 
water  in  the  crucible  over  night,  and  also  to  precipitate  the  trace  of 
lime  in  the  solution  with  ammonium  carbonate  and  filter  off  the 
precipitate.  The  evaporation  of  the  filtrate  containing  the  alkali 
chlorides  can  be  begun  and  at  least  partly  finished  this  day. 

On  Wednesday,  the  fusion  of  the  alumina  precipitate  in  pyro- 
sulphate  (p.  159)  will  be  completed.  The  time  needed  for  this 
is  uncertain,  being  largely  dependent  on  the  amount  of  iron  oxide 
present,  but  it  will  often  be  possible  to  reduce  and  determine  the 
total  iron  oxides  on  the  same  day  that  the  fusion  with  pyrosulphate 
is  begun.  The  alkalies  can  also  be  finished  during  this  day. 

The  calcium  oxalate  precipitate  is  filtered  off,  redissolved  and 
reprecipitated  in  the  morning,  and  the  lime  weighed  after  the  pre- 
cipitate has  stood  for  two  hours  or  so.  Sodium  ammonium  phos- 
phate is  added  to  the  filtrate  to  precipitate  magnesia  (p.  180), 
and  the  beakers  are  allowed  to  stand  over  night. 

On  Thursday,  the  magnesia  precipitate  can  first  be  filtered  off, 
redissolved  and  reprecipitated/  It  can  be  filtered  off,  ignited  and 
weighed  late  in  the  afternoon,  or  this  can  be  done  Friday  morning. 
The  alkalies  are  to  be  entirely  finished  this  day,  if  they  have  not 
been  done  before,  as  should  have  been  easily  possible.  The  evap- 


118  METHODS 

oration  to  dryness  of  the  alkali  chlorides  will  take  an  hour  to  two 
hours  on  the  water-bath;  their  weighing,  fifteen  minutes,  and  the 
evaporation  of  the  platinichloride  solution  should  not  take  more 
than  half  an  hour.  The  filtration  in  a  Gooch  crucible  and  the 
drying  and  weighing  of  the  potassium  platinichloride  ought  to  be 
completed  in  about  an  hour.  The  actual  time  actively  spent  in 
these  operations  is  small. 

The  total  iron  oxides  (p.  162)  can  be  titrated  this  morning,  if 
this  has  not  yet  been  done.  The  time  needed  for  reducing  the 
ferric  to  ferrous  oxide  with  hydrogen  sulphide  will  vary  from  one- 
half  to  three-quarters  of  an  hour,  the  filtration  of  this  solution  may 
take  another  half  hour,  the  expulsion  of  the  hydrogen  sulphide  by 
boiling  and  the  subsequent  cooling  of  the  liquid  should  be  over  in 
less  than  an  hour,  while  the  actual  titration  should  be  done  in 
about  twenty  minutes,  including  the  weighing  of  the  burette 
twice. 

When  the  total  iron  oxides  have  been  determined  the  colori- 
metric  determination  of  titanium  dioxide  (p.  167)  can  be  carried 
out.  All  of  this  operation  should  not  take  more  than  one  one- 
half  or,  at  most,  three-quarters  of  an  hour. 

The  determination  of  ferrous  oxide  should  also  be  done  on  this 
day,  though  time  might  have  been  found  for  it  before,  as  the  oper- 
ation should  not  take  more  than  about  one-half  an  hour,  during 
which,  however,  it  will  need  constant  and  undivided  attention. 

It  should  also  be  possible  to  complete  the  manganese  and  phos- 
phorus determinations  this  day,  and  sulphur  and  baryta  or  zir- 
conia,  if  these  have  been  begun  previously.  Time  can  also  be 
found  for  making  the  total  water  determination,  an  operation  that 
will  consume  little  more  than  half  an  hour. 

The  whole  analysis  should,  therefore,  be  completed  by  Thurs- 
day evening,  but,  if  not,  Friday  morning  should  see  the  completion 
of  the  various  end  determinations,  such  as  the  magnesia,  manga- 
nese, phosphorus,  zirconia,  etc. 

It  will  be  found  that  there  will  be  many  intervals  during  which 
parts  of  the  determinations  of  the  minor  constituents  can  be  car- 
ried out,  as  these  call  for  small  volumes  of  liquid  and  so  can  be 
readily  done  in  the  gaps  between  the  parts  of  the  major  operations. 


ERRORS  AND  SUMMATION  119 


3.  ERRORS  AND  SUMMATION 

Character  of  Errors. — It  is  well  recognized  that  no  analysis 
can  be  ideally  perfect ;  that  is,  yield  results  that  show  the  propor- 
tions of  the  various  constituents  with  absolute  accuracy.  No 
analyst  is  continuously,  to  say  nothing  of  wholly,  perfect  in  his 
manipulations;  no  piece  of  apparatus  is  entirely  free  from  faults 
and  no  reagent  free  from  all  impurities ;  no  conditions  can  be  com- 
pletely controlled;  lastly,  no  method  for  any  determination  is 
known  that  has  not  inherent  in  it  some  source  or  sources  of  error. 
The  best  that  the  analyst  can  do  is  to  strive  to  reduce  to  a  mini- 
mum the  various  errors  that  may  arise. 

In  connection  with  the  descriptions  of  the  various  methods  for 
the  determination  of  the  several  constituents  there  will  be  dis- 
cussed the  particular  errors  inherent  in  each.  It  will  be  well, 
however,  to  present  to  the  student  some  ideas  on  the  general  char- 
acters of  analytical  errors,  so  that  he  may  deal  more  understand- 
ingly  with  the  particular  cases  as  they  arise. 

Analytical  errors  may  be  referred  to  two  broad  groups,  which 
may  be  called  the  "  operative  "  errors  or  errors  of  operation,  which 
are  incident  to  an  operation  or  to  the  manipulation,  and  the 
"  methodic  "  errors  or  errors  of  method,  which  are  inherent  and 
peculiar  to  the  various  methods.1 

The  "  operative  "  errors  are  those  such  as  are  caused  by  the 
entrance  of  dust,  spilling  of  drops  and  other  mechanical  losses,  or 
too  long-continued  washing  of  precipitates.2  The  "  methodic  " 
errors  include  such  as  those  due  to  co-precipitation  of  magnesia 
with  alumina,  the  ready  oxidizability  of  ferrous  compounds,  or 
the  strong  adsorption  of  salts  by  gelatinous  precipitates. 

These  differ  essentially  in  that  the  operative  errors  are  due  to 
causes  outside  the  chemical  and  physico-chemical  factors  of  the 
analysis;  while  the  methodic  errors  are  dependent  on  the  chem- 

1  This  division  is  one  of  practicality  and  differs  from  the  usual  division 
into  "  accidental  "  and  "  systematic  "  errors,  which  are  especially  adapted  to 
mathematical  treatment.     See  Mellor,  Higher  Mathematics  for  Students  of 
Chemistry,  1905,  pp.  502,  529. 

2  Included  among  them  are  also  "  personal  "  errors  and  those  due  to  impur- 
ities in  reagents,  inaccuracy  in  instruments,  and  the  non-determination  of  con- 
stituents.   Operative  errors  are  discussed  on  pp.  75-79. 


120  METHODS 

ical  characters  and  peculiarities  of  the  substances  involved  and 
on  the  conditions  under  which  they  are  made  to  react. 

That  the  one  group  may  grade  into  the  other,  so  to  speak,  is 
shown  by  such  examples  as  the  errors  due  to  the  overwashing  of 
precipitates  and  to  the  adsorption  of  salts,  in  which  it  is  a  case  of 
attaining  correct  results  by  the  balancing  of  operative  manipula- 
tion against  peculiarities  inherent  in*the  method. 

The  occurrence  of  operative  errors  is  brought  about  mostly  by 
carelessness  in  manipulation,  so  that  they  are,  or  should  be, 
almost  entirely  avoidable,  they  should  be  always  eliminated  so 
far  as  possible  by  the  use  of  skill,  dexterity,  cleanliness,  and  thought. 

The  methodic  errors,  on  the  other  hand,  are  inherent  in  the 
particular  methods,  and  so  can  seldom,  if  ever,  be  wholly  elimi- 
nated. They  can,  however,  be  controlled  and  reduced  to  a  mini- 
mum by  sufficient  knowledge  and  consequent  attention  to  the 
proper  conditions  of  reaction. 

Some  of  the  operative  errors,  such  as  the  entrance  of  dust  or 
the  spilling  of  drops,  are  so  obvious  that  nothing  need  be  said  of 
them  to  the  intelligent  and  conscientious  student.  Others  have 
been  briefly  dealt  with  in  the  section  on  operations,  and  some  pre- 
cautions have  been  pointed  out  by  which  they  may  be  avoided. 
In  the  subsequent  part  of  this  book,  in  general,  the  operative  errors 
will  be  assumed  to  be  known  and  guarded  against,  and  only  those 
of  a  methodic  character  will  be  considered  and  discussed. 

Direction  of  Errors. — The  characters  of  the  probable  syste- 
matic errors  to  which  the  determinations  of  the  several  more 
important  constituents  are  subject  will  be  discussed  in  connection 
with  the  descriptions  of  the  various  methods.  It  may  be  of  interest 
and  use  to  the  student  to  summarize  very  briefly  the  probable 
direction  of  methodic  error  1  of  the  more  important  constituents, 
whether  plus  or  minus;  that  is,  whether,  because  of  them,  a  care- 
ful worker  may  be  prepared  to  expect  a  result  to  be  higher  or  Jower 
than  the  truth. 

Dittrich2  has  investigated  this  subject  as  regards  alumina, 
ferric  oxide,  lime,  magnesia,  soda,  and  potash.  Some  of  his 

1  It  is  understood  that  such  operative  errors  as  those  due  to  spilling  of 
drops,  overwashing  of  precipitates,  entrance  of  dust,  impurities  in  reagents, 
and  many  others,  will  not  be  considered  here. 

2  M.  Dittrich,  Neu.  Jahrb.,  2,  p.  69,  1903. 


ERRORS  AND  SUMMATION  121 

methods  were  poor  and  one-half  of  the  analyses  were  carried  out 
by  incompetent  assistants,  so  that  his  results  are  of  little  value. 
Robinson  1  has  tabulated  the  probable  direction  of  error  for  the 
more  important  constituents,  basing  his  conclusions  on  many 
analyses  made  by  several  analysts  according  to  the  methods  given 
by  Hillebrand.  Some  very  instructive  comments  on  a  large  series 
of  analyses  of  samples  of  the  same  argillaceous  limestone  are  given 
by  the  Committee  on  Uniformity  in  Technical  Analysis.2 

The  general  conclusions  to  which  I  have  been  led,3  independ- 
ently of  this  report,  by  consideration  of  the  methods  employed 
and  from  critical  study  of  very  many  analyses  of  igneous  rocks  are 
as  follows: 

Silica. — The  errors  incident  to  this  determination  are,  on  the 
whole,  of  small  magnitude  and  with  a  tendency  to  the  minus  side. 
If  a  second  evaporation  for  silica  is  made,  and  that  which  is  dis- 
solved in  pyrosulphate  is  recovered,  they  may  be  regarded  as 
negligible;  the  more  so  as  silica  is,  in  the  vast  majority  of  rocks, 
the  constituent  that  is  present  in  the  largest  amount,  and  its 
determination  does  not  affect  that  of  others.  In  judging  analyses 
of  inferior  quality  I  have  found  that  the  determination  of  silica 
(with  that  of  lime)  is  the  one  in  which  most  confidence  can  be 
placed. 

Alumina. — This  constituent  is  more  subject  to  error,  both  in 
magnitude  and  variety,  than  any  other  of  those  determined  in 
all  rock  analyses.  These  errors  may  be  of  very  serious  conse- 
quence, despite  alumina  being  usually  the  most  abundant  con- 
stituent (next  to  silica),  as  its  correct  determination  is  of  great 
importance  in  many  mineralogical  calculations. 

The  errors  for  alumina  in  the  aggregate  tend  to  be  plus; 
though  there  are  a  few  minus  ones,  these  are  generally  much 
more  than  outweighed  by  those  that  are  plus.  This  plus  tendency 
is  increased  in  incomplete  analyses  by  the  indirect  determination 
of  alumina  by  difference,  so  that  the  neglect  of  any  of  the  con- 
stituents that  are  weighed  with  it  will  increase  its  apparent  amount. 

In  the  hands  of  incompetent  or  inexpert  workers,  the  two  chief 
sources,  non-determination  of  constituents  that  are  weighed  with 

1  H.  H.  Robinson,  Am.  Jour.  Sci.,  41,  p.  259,  1916. 

2  Jour.  Am.  Chem.  Soc.,  28,  p.  232,  1906. 

3  Washington,  Prof.  Paper  99,  pp.  13,  20. 


122  METHODS 

alumina  (p.  8)  and  the  co-precipitation  of  magnesia,  as  well  as 
the  difficulty  of  washing  the  precipitate,  with  the  determination  of 
alumina  by  difference,  cause  the  figure  for  alumina  to  be  almost 
invariably  and  inevitably  high — often  by  several  or  many  per  cent. 
Even  in  the  hands  of  careful  workers,  though  alumina  can  be 
determined  with  almost  as  much  accuracy  as  can  most  of  the  other 
constituents,  there  is  a  liability  to  variation  or  irregularity,  with  a 
plus  tendency. 

The  determination  of  alumina  must  be  regarded  as  the  most 
unsatisfactory  of  all,  even  for  the  expert  analyst,  and,  because  of 
its  importance,  an  accurate  method  for  its  direct  determination  is 
the  most  urgent  need  in  the  analysis  of  rocks. 

Ferric  Oxide. — Failure  to  reduce  the  ferric  oxide  completely 
to  ferrous  and  loss  through  "  creeping  "  in  the  pyrosulphate  fusion 
tend  to  minus  errors,  while  reduction  with  zinc  and  deterioration 
of  the  permanganate  solution  have  the  opposite  effect.  These 
main  sources  of  error,  however,  should  not  be  of  great  magnitude 
if  the  work  is  carefully  done.  In  general,  I  think  that  the  ten- 
dency with  ferric  oxide  is  toward  minus,  rather  than  plus  error, 
and  that  it  is  seldom  serious  with  a  careful  worker.  If  zinc  has 
been  used  for  the  reduction  it  is  almost  certainly  plus. 

Ferrous  Oxide. — This  has  always  been,  and  probably  will 
remain,  the  direct  determination  in  rock  analysis  that  is  most 
fraught  with  uncertainty  and  difficulty  in  obtaining  very  accurate 
results.  These  arise,  for  the  most  part,  from  the  ready  oxidiza- 
bility  of  the  constituent  and  the  difficulty  in  bringing  it,  unoxidized, 
into  solution  from  the  not  easily  soluble  or  decomposable  silicates. 
This  liability  to  oxidation  and  difficulty  of  decomposition  tend,  of 
course,  towards  low  results.  The  only  important  factors  of  oppo- 
site tendency  are  the  evanescent  and  uncertain  end-point  pro- 
duced by  hydrofluoric  acid  and  the  influence  of  manganous  oxide 
in  the  permanganate  titration.  The  plus  tendency  caused  by 
organic  matter,  sulphides,  and  vanadium  may  be  disregarded 
with  most  rocks.  On  the  whole,  there  is  a  strong  tendency  to 
minus  error  in  the  determination  of  ferrous  oxide. 

Lime. — If  the  ammonia  water  is  free  from  carbonate  and  the 
calcium  oxalate  is  dissolved  and  reprecipitated,  there  are  no  serious 
systematic  errors  in  the  determination  of  lime.  Indeed,  the  figures 
for  lime,  as  well  as  silica,  can  generally  be  taken  with  confidence 


ERRORS  AND  SUMMATION  123 

as  to  their  approximate  accuracy,  even  in  otherwise  rather  poor 
work.  The  error  may  be  considered  as  having  a  very  slight  plus 
tendency,  so  small  as  to  be  negligible. 

Magnesia. — Unless  the  analytical  work  is  very  careless,  there 
should  be  little  serious  error  in  the  determination  of  magnesia. 
The  liability  to  precipitation  of  an  ammonium  magnesium  phos- 
phate of  abnormal  and  variable  composition,  which  tends  to  plus 
errors,  is  readily  eliminated  by  solution  and  reprecipitation  under 
proper  conditions.  The  retention  of  magnesia  with  the  alumina, 
due  to  paucity  of  ammonium  salt,  is  possible  or  probable  with 
ignorant,  careless,  or  hasty  work,  of  which,  indeed,  it  may  be  con- 
sidered to  be  one  of  the  characteristics.  This  minus  error  may  be 
of  very  serious  magnitude.  With  good  analysts  the  error  in  mag- 
nesia may  be  considered  as  plus  or  minus,  that  is,  probably  negli- 
gible; while  with  inferior  workers  it  is  almost  always  plus,  and 
often  very  highly  so. 

Potash. — With  both  potash  and  soda,  if  the  Smith  method  of 
decomposition  is  used  (as  is  assumed),  there  is  some  tendency  to 
minus  error,  because  of  retention  of  alkali  chloride  in  the  fusion 
cake  through  incomplete  leaching.  This  should,  however,  be  very 
small.  The  plus  error  due  to  alkali  in  the  calcium  carbonate  is 
easily  eliminated  by  making  the  proper  correction,  as  should 
always  be  done  if  the  carbonate  is  of  such  poor  quality  as  to  call 
for  it.  Errors  due  to  other  causes  are  so  small  in  good  work  as  to 
be  negligible.  On  the  whole  the  error  tendency  in  potash  may  be 
regarded  as  negligible,  but  with  a  slight  leaning  toward  minus. 

Soda. — The  remarks  just  made  in  connection  with  potash,  as 
to  the  errors  due  to  incomplete  washing  and  to  alkali  in  the  car- 
bonate, apply  equally  well  to  soda.  As  the  amount  of  soda  is 
generally  greater  than  that  of  potash,  and  as  it  is  determined  by 
difference,  these  errors  may  be  somewhat  magnified  with  soda. 
The  general  tendency  is  the  same  as  with  potash.  In  my  experi- 
ence I  have  found  both  the  alkalies  to  be  among  the  most  accu- 
rately and  consistently  determinable  of  the  rock  constituents. 

Water. — Unless  the  rock  is  high  in  ferrous  oxide  and  the  water 
is  determined  "  on  ignition,"  there  are  no  notable  errors  in  the 
determination  of  water,  either  "  combined  "  or  "  hygroscopic." 
The  amounts  are  usually  so  small,  and  this  constituent  is  as  a  rule 
so  unimportant,  that  the  errors  may  be  regarded  as  insignificant. 


124  METHODS 

Titanium  Dioxide. — If  the  colorimetric  method  is  used,  the 
errors  in  this  determination  are  small  and,  because  of  the  generally 
small  amount  present,  are  seldom  serious.  Because  of  the  bleach- 
ing effects  of  hydrofluoric  acid  and  alkali  sulphates,  as  has  been 
pointed  out  by  Merwin,  there  is  a  tendency  toward  small  minus 
error.  If  the  antiquated  methods  based  on  boiling  acid  solutions 
be  used  the  errors  are  variable  in  direction  and  may  be  relatively 
large;  they  are  probably  generally  plus. 

Phosphorus  Pentoxide. — The  errors  incident  to  the  determina- 
tion of  this  are  so  seldom  serious,  and  the  usur1  amounts  of  this 
constituent  are  in  most  rocks  so  small,  that  they  may  be  considered 
as  insignificant.  The  tendency,  if  any,  is  toward  very  slight  plus 
error. 

Manganous  Oxide. — If  the  colorimetric  method  is  used  the  errors 
in  the  determination  of  manganese  are  negligible,  particularly  as 
the  amount  present  is  almost  always  very  small.  If  the  basic 
acetate  method  is  used  and  the  analyst  is  not  expert,  there  is  a 
decided  tendency  to  plus  error,  which  may  be  of  both  relatively 
and  absolutely  large  magnitude.1 

In  the  determination  of  the  other  minor  constituents  the  pos- 
sible errors,  though  sometimes  relatively  great,  are  usually  of  such 
slight  absolute  importance  that  their  directions  need  not  be  con- 
sidered. As  Hillebrand  remarks  of  the  rarer  constituents:  "  It 
is  often  more  important  to  know  whether  or  not  an  element  is 
present  than  to  be  able  to  say  that  it  is  there  in  amount  of  exactly 
0.02  or  0.06  per  cent." 

Limit  of  Error.2 — Inasmuch  as  no  determination  can  be  ex- 
pected to  yield  perfectly  accurate  results,  or  to  be  exactly  repro- 
ducible on  duplicate  analysis,  except  by  chance,  we  must  assume 
some  limits  for  the  possible  variation  on  either  side  of  the  truth, 
such  that  values  falling  within  them  may  be  regarded  as  satisfac- 
tory and  consistent  with  good  work.  These  limits  may  be  called 
"  allowable." 

In  attempting  to  allot  the  allowable  limit  of  error  for  each 
constituent,  regard  must  be  had  to  its  amount  in  any  given  case. 
Assuming  that  the  allowable  total  error  is  ±0.60,  which  is  not 

1  Cf.  Hillebrand  et  al.,  Jour.  Am.  Chem.  Soc.,  28,  p.  233,  1906. 

2  Cf.  Hillebrand,  p.  27;  Mellor,  p.  247;  Dittrich,  Neues  Jahrbuch,  2,  p.  69, 
1903;  Hillebrand  et  al,  Jour  Am.  Chem.  Soc.,  28,  pp.  223  ff,  1906. 


ERRORS  AND  SUMMATION  125 

quite  correct,  but  near  enough  for  the  present  purpose,  we  might 
allot  this  proportionately  among  the  chief  constituents  some- 
what as  follows:  Taking,  for  example,  the  average  igneous 
rocks  as  calculated  by  Clarke1  we  would  obtain  these  figures: 
Si02  ±0.35,  A12O3,  ±0.10,  Fe2O3  ±0.02,  FeO,  MgO,  CaO  and 
Na2O  ±0.03,  K2O  ±0.02,  H2O,  TiO2,  P2O5  and  Mno  ±0.01. 
These  are  based  on  the  assumptions  that  the  errors  may  be  all  in 
one  direction  and  are  proportional  to  the  amount  of  each  con- 
stituent. 

We  cannot,  however,  always  expect  such  close  agreement  in 
duplicate  determinations  of  the  less  abundant  constituents  as  is 
implied  by  these  figures.  The  matter  is  further  complicated  by 
the  varied  differences  in  difficulty  and  possible  exactness  that 
condition  the  several  determinations,  particularly  as  some  of  the 
errors  may  probably  compensate  for  each  other  to  some  extent. 

Without  going  into  a  full  discussion  of  this  subject,  and  dis- 
regarding the  various  details  that  are  peculiar  to  the  several  con- 
stituents, we  may  provisionally  assume  the  figures  given  below  as 
the  allowable  limits  of  error  for  constituents  that  are  present  in 
about  the  amounts  mentioned. 

It  will  be  understood  that  the  limits  mentioned  are  in  per- 
centages of  the  whole  rock,  not  of  the  amount  of  each  constituent. 

These  allowable  limits  are :  for  SiO2  and  others  that  amount  to 
30  per  cent  or  over,  from  ±0.10  to  ±0.15;  for  Al2Os  and  others 
that  amount  to  from  10  to  30  per  cent,  ±0.05  to  ±0.10;  for  con- 
stituents that  amount  to  from  1  to  10  per  cent,  ±0.03  to  ±0.05. 

These  figures  are  but  approximate  suggestions,  based  on  experi- 
ence in  analysis  and  in  the  critical  judging  of 'analyses.  They  mean 
that  duplicate  determinations  should  not  differ  from  each  other 
by  more  than  these  amounts,  while  it  is  very  desirable,  and  usually 
quite  possible  in  good  work,  that  they  fall  well  within  them.2 

The  difference  may,  under  peculiar  conditions  or  with  partic- 
ular rocks  or  minerals,  and  with  certain  methods,  be  somewhat 
greater  than  those  mentioned,  without  reflecting  seriously  or  at 

1  F.  W.  Clarke,  U.  S.  Geol.  Surv.,  Bull.  616,  p.  27,  1916. 

2  The  student  will  find  many  such  examples  among  the  mineral  analyses 
of  Penfield  and  many  others  in  the  3d  and  4th  series  of  the  American  Journal 
of  Science;  and  some  of  rock  analyses  in  U.  S.  Geol.  Survey,  Prof.  Paper  99. 
An  excellent  example,  with  comments  on  other  work  on  the  same  material,  is 
that  on  p.  229,  Jour.  Am.  Chem.  Soc.,  28,  1906. 


126  METHODS 

all  on  the  quality  of  the  analysis;  but  such  cases  are  to  be  judged 
only  by  the  experienced  analyst.  The  student  should  not  think 
that  the  somewhat  liberal  latitude  here  given  in  these  allowable 
limits  of  error  justifies  him  in  taking  advantage  of  them  as  an 
excuse  for  poor  work.  He  should,  on  the  contrary,  endeavor  to 
make  his  analyses  so  that  the  differences  between  duplicate  deter- 
minations, if  .they  are  made,  fall  well  within  the  limits  thus  allowed. 

In  order  to  check  his  errors,  and  so  be  in  a  position  to  correct 
them,  the  novice  should  make  duplicate  analyses  throughout,  until 
he  becomes  familiar  with  the  methods  and  the  manipulations,  and 
by  repeated  close  agreements  may  place  justifiable  confidence  in 
his  single  determinations.  This  will,  at  first,  involve  more  labor 
and  the  turning  out  of  fewer  analyses  in  a  given  time;  but  the  in- 
creased value  of  the  results  will  much  more  than  compensate  for 
this  in  the  end.  An  analysis  in  which  the  analyst  himself  cannot 
place  implicit  confidence  is  not  only  of  little  use,  but  is  positively 
dangerous,  for  others,  to  whom  there  may  be  evident  no  reason  for 
doubting  the  data;  and  such  work  will  eventually  and  inevitably 
reflect  injuriously  on  its  maker. 

As  regards  duplicate  analyses,  however,  it  must  be  remem- 
bered that  close  correspondence  in  two  determinations  by  the  same 
method  is  not,  in  itself,  conclusive  proof  of  correctness.  It  is 
possible  to  obtain  closely  concordant  or  practically  identical 
results  on  repetition  by  poor  as  well  as  by  good  methods;  for  if 
the  same  errors  are  made,  and  to  about  the  same  amount,  in  dupli- 
cate analyses,  the  figures  in  each  may  agree  closely  and  yet  be  far 
from  the  truth. 

At  the  same  time,  when  poor  methods  are  used  or  the  analyst 
is  incompetent  the  chances  are  decidedly  against  obtaining  dupli- 
cate results  that  are  so  closely  concordant  as  to  be  satisfactory, 
particularly  if  errors  in  manipulation  have  been  committed;  so 
that,  if  the  methods  are  good  and  the  analyst  is  competent,  dupli- 
cate figures  that  agree  well  with  each  other  justify,  on  the  whole, 
a  high  degree  of  confidence  in  their  correctness. 

Summation.1 — In  the  ideally  perfect  analysis,  of  course,  the 

1  Various  phases  of  this  topic  have  been  discussed  by:  Fresenius,  2,  p.  168; 
Hillebrand,  p.  27;  Mellor,  p.  245;  M.  F.  Connor,  C.  R.  12,  Cong.  Geol.  Int., 
p.  889;  H.  H.  Robinson,  Am.  Jour.  Sci.,  46,  p.  257,  1916;  H.  S.  Washington, 
Prof.  Paper  99,  p.  21. 


ERRORS  AND  SUMMATION  127 

summation  will  be  exactly  100  per  cent;  but  in  practice,  as  is 
well  recognized,  this  result  is  seldom  obtained,  and  if  so  it  must 
usually  be  regarded  as  due  to  the  compensation  of  different  plus 
and  minus  errors. 

As  Hillebrand  has  stated,  "  A  complete  silicate  rock  analysis 
which  foots  up  less  than  100  per  cent  is  generally  less  satisfactory 
than  one  which  shows  a  summation  somewhat  in  excess  of  100. 
This  is  due  to  several  causes.  Nearly  all  reagents,  however 
carefully  purified,  still  contain,  or  extract  from  the  vessels  used, 
traces  of  impurities,  which  are  eventually  weighed  in  part  with 
the  constituents  of  the  rock.  The  dust  entering  an  analysis 
from  first  to  last  is  very  considerable,  washings  of  precipitates 
may  be  incomplete,  and  if  large  filters  are  used  for  small  pre- 
cipitates the  former  may  easily  be  insufficiently  washed." 

On  the  other  hand,  deficiencies  in  the  summation  may  be 
caused  by  mechanical  loss,  such  as  through  spilling  of  drops  or 
blowing  away  of  light  powders;  or  by  physico-chemical  factors,  as 
the  partial  solution  of  slightly  soluble  precipitates,  or  the  incom- 
plete absorption  of  carbon  dioxide  or  water.  A  low  summation 
may  also  be  caused  by  the  non-determination  of  some  constituent. 

The  correctness  of  the  opinion  of  Hillebrand  and  other  experi- 
enced analysts  that  the  plus  errors  tend  to  surpass  the  minus  errors 
is  shown  clearly  by  Robinson's  average  summation,  derived  from 
3391  analyses.  This  is  100.13,  with  a  maximum  at  100.15- 
100.19. 

The  limits  of  summation  below  or  above  100  per  cent  which 
may  be  considered  allowable  and  consistent  with  satisfactory  work 
are  considered  by  Hillebrand  to  be  99.75  and  100.50,  and  by  Mellor, 
99.50  and  100.50.  For  allowable  limits  appropriate  to  first-class 
work  by  an  experienced  analyst  I  am  in  accord  with  Hillebrand. 
But  for  the  usual  run  of  analytical  work  one  may  be  liberal  and 
fairly  extend  these  limits  to  99.50  and  100.75,1  though  the  lower 
limit  is,  as  Hillebrand  remarks,  rather  too  low  for  first-class  work, 
especially  in  view  of  the  tendency  to  high  summations. 

If  the  analyst  obtains  a  summation  between  these  limits  he 

may  consider  his  results  as  satisfactory,  provided  that  there  is  no 

reason  to  suspect  errors  having  been  made  that  compensate  each 

other.     For  the  student  must  realize  that  a  summation  of  nearly 

1  Cf.  Washington,  Prof.  Paper  99,  p.  21. 


128  METHODS 

or  exactly  100  per  cent  is  not  conclusive  evidence  of  accurate  work, 
because  of  this  possible  balancing  of  plus  and  minus  errors. 

If  the  analysis  foots  up  under  the  lower  limit,  especially  in 
several  analyses  of  a  series  of  similar  rocks,  there  is  strong  prob- 
ability that  some  constituent  has  been  overlooked  or  some  sys- 
tematic error  has  been  committed.  In  this  case,  or  if  the  summa- 
tion is  above  100.75,  the  analysis  should  be  repeated  in  whole  or 
in  part.  As  Hillebrand  remarks:  "  It  is  not  proper  to  assume  that 
the  excess  (or  deficiency)  is  distributed  over  all  the  determined 
constituents.  It  is  quite  as  likely,  in  fact  more  than  likely,  to 
affect  a  single  determination  and  one  which  may  be  of  importance 
in  a  critical  study  of  the  rock  from  the  petrographic  side." 

There  are  several  special  cases  of  high  or  low  summation  that 
are  connected  with  the  determination  of  various  constituents,  and 
which  do  not,  in  themselves,  indicate  inferiority  of  the  analysis 
as  a  whole. 

If  water  be  determined  by  loss  on  ignition  the  summation  will 
usually  be  lower  than  it  would  be  were  the  water  determined 
directly.  This  is  because  of  the  partial  oxidation  of  the  ferrous 
oxide  in  the  rock,  and  a  consequent  gain  in  weight,  the  algebraic 
sum  of  this  and  the  actual  loss  of  water  producing  an  apparent 
amount  of  water  less  than  that  which  is  really  present. 

If  the  iron  oxides  are  not  separately  determined,  but  are  given 
as  ferric  oxide,  the  summation  will  be  too  high  by  one-ninth  of  the 
amount  of  ferrous  oxide  present;  and,  conversely,  if  they  are 
given  as  ferrous  oxide  alone,  the  summation  will  be  too  low  by  one- 
tenth  of  the  ferric  oxide  present.1  This  error  is,  of  course,  elim- 
inated if  both  oxides  are  determined  correctly. 

If  the  analysis  shows  that  chlorine,  fluorine,  or  sulphur  (as 
sulphide)  are  present,  an  amount  of  oxygen  equivalent  to  the 
amounts  of  these  must  be  deducted,  or  the  summation  of  the  anal- 
ysis will  be  too  high  by  that  amount.  The  oxygen  equivalent  of 
chlorine  is  0.22  of  its  amount,  that  of  fluorine  0.42,  and  that  of 
sulphur  is  0.43  if  this  is  present  only  in  pyrrhotite.  As  regards 
the  sulphur  of  pyrite,  the  iron  with  which  it  is  combined  will  be 
given  as  ferric  oxide  in  the  statement  of  the  analysis,  although 

1  The  molecular  weight  of  Fe2O3  is  160  and  that  of  2FeO  is  144,  the  dif- 
ference (the  weight  of  one  atom  of  oxygen)  being  one-tenth  of  the  former  and 
one-ninth  of  the  latter. 


WEIGHING  OUT  THE  PORTIONS  129 

Hillebrand  has  shown  that  the  mineral  is  attacked  by  sulphuric 
and  hydrofluoric  acids  to  only  a  scarcely  appreciable  extent  in  the 
determination  of  ferrous  oxide.  Consequently  the  oxygen  equiva- 
lent of  sulphur  in  pyrite  is  0.375,  instead  of  0.25,  as  it  would  be 
were  its  iron  content  determined  as  ferrous  oxide. 

To  give  an  example  of  the  application  of  these  corrections: 
if  the  sum  of  an  analysis  is  100.28  and  there  is  0.54  Cl  present,  we 
must  deduct  0.54X0.22  =  0.12,  leaving  100.16  as  the  correct  sum- 
mation. The  corrections  for  fluorine  and  sulphur  will  seldom  be 
called  for. 

In  the  earlier  days  of  analysis  both  chemists  and  petrographers 
were  content  with  very  poor  summations,  even  with  those  which 
fell  below  99.00  or  above  101.00.  It  is  to  be  regretted  that  the 
same  complacency,  though  less  often  met  with,  is  not  quite  extinct 
at  the  present  time.  The  intelligent  and  conscientious  analyst  or 
petrographer  should  look  upon  such  summations  with  the  gravest 
suspicion,  and  reject  or  remake  any  analysis  that  thus  furnishes 
evidence  of  such  manifestly  erroneous  results,  either  throughout 
or  in  part  of  the  analysis.1 

4.  WEIGHING  OUT  THE  PORTIONS 

There  are  two  methods  of  weighing  out  portions  of  rock  powder 
for  analysis;  these  may  be  called  the  method  by  addition  and  the 
method  by  subtraction. 

The  method  by  addition  is  most  generally  used  and  serves  best 
for  all  the  rock  portions,  except  that  used  for  the  determination  of 
the  alkalies.  The  previously  ignited  and  cooled  crucible  (covered) 
is  weighed  to  tenths  of  a  milligram  as  described  previously,  and  its 
weight  is  recorded.  A  weight  equal  to  that  desired,  say  1  gram, 
is  added  to  the  right-hand  pan.  The  crucible  and  its  cover  are 
removed  from  the  left-hand  pan  with  the  forceps,  the  crucible 
(uncovered)  being  placed  on  the  table  in  front  of  the  balance 
case.  A  little  of  the  rock  powder  is  carefully  removed  from  the 
specimen  tube  with  the  platinum  spatula  and  placed  in  the  cru- 
cible, taking  care  to  raise  as  little  dust  as  possible  and  that  none 
of  the  powder  adheres  to  the  sides  of  the  crucible  The  crucible 

1  This  subject  is  discussed  at  some  length  in  Washington,  Prof.  Paper  99, 
pp.  21,  24,  25. 


130  METHODS 

is  then  replaced  on  the  left-hand  pan,  its  cover  laid  on,  and  the 
pans  and  arms  gently  released.  If  there  is  not  enough  powder  to 
slightly  more  than  outweigh  the  added  weight,  a  little  more  is 
added,  or  if  there  is  too  great  an  excess  of  powder,  a  little  is  taken 
out  with  the  platinum  spatula;  the  crucible  being,  in  either  case, 
removed  from  the  pan  and  placed  on  the  table,  so  that  none  of  the 
powder  may  fall  upon  the  pan.  This  is  continued  until  there  is  in 
the  crucible  a  weight  of  powder  but  slightly  (say  about  1  centigram) 
more  than  the  added  weight.  The  correct  weight  is  then  taken  to 
one-tenth  of  a  milligram  as  before.  With  a  little  experience  one  is 
soon  able  to  judge  quite  well  when  there  is  about  the  right  amount 
of  powder,  allowance  being  made  for  the  different  specific  gravi- 
ties of  different  rocks.1  Also,  one  will  soon  be  able  to  judge  from 
the  movements  of  the  pointer  whether  the  difference  in  weights 
on  either  pan  is  great  or  not. 

The  method  by  subtraction  is  used  for  weighing  out  the  por- 
tion for  the  alkali  determination,  as  well  as  when  substances  have 
to  be  weighed  out  accurately  to  make  up  standard  solutions,  as 
with  sodium  oxalate.  The  specimen  tube  containing  the  powder 
is  wiped  perfectly  dry  and  uncorked,  and  is  then  placed  on  the 
left-hand  pan  of  the  balance.  It  is  well  to  support  it  in  a  light 
metal  tube  stand,  so  as  to  prevent  its  rolling.  In  handling  the 
tube  during  the  weighing  it  is  scarcely  necessary  to  use  a  dry 
handkerchief  or  test-tube  holder,  as  little  or  no  appreciable  error 
is  introduced  if  the  fingers  are  perfectly  dry. 

When  it  has  been  weighed,  the  requisite  weight  (say  one-half  a 
gram)  is  removed  from  the  right-hand  pan,  and  an  amount  of 
powder  about  equal  to  this  is  very  carefully  poured  out  into  the 
proper  receptacle,  which  will  be  the  platinum  basin  with  the  alkali 
determination  (p.  195).  This  pouring  must  be  done  with  the 
greatest  care,  and  the  mouth  of  the  tube  is  to  be  held  close  to 
the  bottom  of  the  basin,  so  as  to  prevent  loss  of  powder.  During 
the  pouring,  also,  the  breathing  should  not  be  directed  toward  the 
basin,  or  some  of  the  powder  may  be  blown  out. 

When  about  a  sufficient  amount  has  been  poured  out2  the 

1  Thus,  a  one-gram  heap  of  basalt  powder  is  distinctly  smaller  than  one  of 
granite. 

2  The  beginner  would  better  proceed  cautiously  and  pour  out  at  first  what 
is  evidently  too  little. 


FUSION  WITH  SODIUM   CARBONATE  131 

mouth  of  the  tube  is  tilted  up  (still  above  the  basin),  and  the 
sloping  tube  is  gently  turned  around  on  its  axis,  and  possibly 
tapped  very  lightly,  so  as  to  bring  the  remaining  powder  away  from 
the  mouth  and  toward  the  bottom  of  the  tube  without  loss.  The 
basin  is  covered  with  a  watch-glass,  and  the  tube,  still  uncorked, 
is  weighed  as  before.  The  loss  in  weight  is  the  weight  of  powder 
taken  for  analysis. 

It  may  be  necessary  to  pour  out  several  additional  small  por- 
tions so  as  to  get  the  amount  that  is  needed.  A  small  excess,  say 
of  a  few  centigrams,  or  even  a  decigram,  will  be  of  no  consequence 
in  the  alkali  determination.  But  if  one  has  poured  out  too  great 
an  excess,  this  cannot  be  corrected  by  replacing  some  of  the  powder 
in  the  tube,  because  some  of  it  would  inevitably  be  lost.  If  this 
happens  all  of  the  powder  is  to  be  replaced  in  the  tube,  the  basin 
is  wiped  clean  and  dry,  and  the  operation  is  begun  over  again. 

5.  FUSION  WITH  SODIUM  CAKBONATE  l 

For  the  determination  of  silica,  alumina,  total  iron  oxides, 
titanium  dioxide,  lime  and  magnesia,  in  what  is  called  the  "  main 
portion,"  decomposition  may  be  effected  by  several  fluxes,  as  has 
been  mentioned  on  p.  85.  Of  these,  sodium  carbonate  is,  for 
general  use,  by  far  the  best  and  the  one  most  often  used.  It  is 
used  exclusively,  indeed,  for  this  purpose  by  the  chemists  of  the 
U.  S.  Geological  Survey  and  by  myself.  It  will,  therefore,  be  the 
only  one  to  be  considered  here. 

The  Fusion. — The  method  of  fusion  with  sodium  carbonate 
owes  its  usefulness  to  the  fact  that  this  reagent  at  the  temperature 
of  fusion  decomposes  the  minerals  present,  forming  silicate,  alum- 
inate,  titanate,  phosphate  and  zirconate  of  sodium,  and  carbonates, 
silicates,  and  possibly  aluminates,  of  iron,  manganese,  magnesium, 
calcium  and  barium,  all  of  which  are  readily  soluble  in  hydrochloric 
acid. 

About  1  gram  of  rock  powder  is  generally  used  for  this  opera- 
tion. A  platinum  crucible  of  35  or  40  c.c.  capacity  is  selected.  A 
smaller  one  is  not  appropriate  on  account  of  danger  of  loss  through 
bubbling  of  the  melted  mass,  as  well  as  on  account  of  greater  dif- 

1  Classen,  2,  p.  608;  Hillebrand,  pp.  87-90;  Mellor,  pp.  163-166;  Ostwald, 
p.  218;  Treadwell,  2,  pp.  488-489. 


132  METHODS 

ficulty  in  loosening  the  solid  cake.  For  this  fusion,  the  sides  of 
the  crucible  should  have  considerable  flare,  and  one  with  vertical 
sides  is  unsuitable.  The  crucible  used  for  this  fusion  must  be 
hard  and  not  easily  dented  or  bent.  It  should,  therefore,  be  of 
platinum  alloyed  with  iridium  or  of  palau  (which  I  have  found  to 
answer  very  well),  but  not  of  pure  platinum,  which  is  much  too 
soft.  Any  loss  in  weight  upon  ignition  is  of  no  consequence  here, 
as  the  crucible  is  not  weighed  both  before  and  after  fusion,  and  it 
may  be  reserved  for  this  purpose. 

The  crucible  is  cleaned,  ignited  to  bright  redness,  placed  in  the 
desiccator  after  it  has  cooled  below  a  red  heat,  and  allowed  to  cool. 
When  perfectly  cold,  it  is  weighed  with  the  cover  on,  the  weighing 
being  carried  to  tenths  of  a  milligram  by  means  of  the  rider,  and 
the  weight  noted.  This  weighing  is  carried  out  according  to  the 
directions  given  on  pp.  129-131,  and  the  weighing  out  of  the 
powder  is  carried  out  in  the  manner  just  described. 

It  is  not  necessary,  indeed  it  is  better  not,  to  weigh  out  exactly 
1  gram,  which  will  take  considerable  time,  but  an  amount  varying 
from  0.9  to  1.1  gram  should  be  taken,  preferably  a  little  more  than 
a  little  less  than  a  gram.  With  some  practice  it  will  be  found  sim- 
ple to  estimate  with  the  eye  when  one  has  about  the  right  amount. 

The  crucible  (covered)  and  the  weights  being  removed  from 
the  balance,  one  of  a  pair  of  balanced  3-inch  watch-glasses  is 
placed  on  the  right-hand  pan,  and  a  5-gram  weight  placed  on 
it.  On  the  other  watch-glass,  dry,  powdered,  anhydrous  sodium 
carbonate  is  placed  by  means  of  a  dry  horn  spoon,  which  is  kept 
for  this  purpose  in  the  balance-case  drawer,  and  which  must  be 
carefully  wiped  off  at  the  end  of  the  operation.  Enough  is  added 
or  subtracted  to  balance  the  other  watch-glass  and  the  5-gram 
weight.  The  addition  or  subtraction  is  to  be  done  with  the  watch- 
glass  on  the  table,  not  on  the  pan,  lest  some  of  the  carbonate  get 
on  the  latter.  It  is  not  necessary  to  weigh  the  carbonate  accu- 
rately, but  the  difference  should  not  be  more  than  a  few  decigrams 
either  way.  It  is  usually  stated  that  the  amount  of  carbonate 
should  be  four  times  that  of  the  substance  taken,  but  it  is  found 
that  a  somewhat  larger  amount  is  advisable  for  proper  fusion,  and 
for  very  basic  rocks  as  much  as  6  grams  may  be  taken  advan- 
tageously, as  with  these  the  decomposition  is  less  easy. 

The  crucible  is  placed  on  a  clean  sheet  of  paper,  the  cover 


FUSION  WITH  SODIUM   CARBONATE  133 

laid  to  one  side,  and  the  greater  part  of  the  sodium  carbonate 
is  transferred  to  the  crucible  by  means  of  the  platinum  spatula, 
care  being  taken  that  none  of  the  rock  powder  is  thrown  out. 
About  half  a  gram  of  carbonate  should  be  left  on  the  watch-glass. 

The  mixing  of  the  rock  powder  with  the  carbonate  and  the 
process  of  fusion  have  been  described  on  (pp.  86-87).  As, 
however,  this  fusion  is  of  primary  importance,  it  will  be  well  to 
repeat  here  the  description  of  the  operation. 

The  flux  and  powder  in  the  crucible  are  thoroughly  mixed  by 
stirring  them  gently  with  the  broad  end  of  the  dry  platinum  spat- 
ula, this  being  done  so  that  there  is  no  loss  of  powder.  They 
should  be  so  intimately  mixed  that  the  mass  appears  homogeneous 
to  the  eye,  and  particular  care  should  be  paid  to  getting  the  car- 
bonate well  down  and  around  all  the  corners,  so  that  no  patches  of 
unmixed  rock  powder  remain  at  the  bottom,  where  they  would 
be  attacked  slowly  and  with  difficulty. 

When  the  powders  are  intimately  mixed  and  smoothed  down, 
the  spatula  is  cleaned  of  any  adherent  mixture  by  rubbing  on  the 
portion  of  carbonate  that  remains  on  the  watch-glass.  This 
portion  is  then  added  to  the  mixture  in  the  crucible. 

The  covered  crucible  is  then  placed  vertically  in  a  triangle,  and 
is  heated  at  a  height  of  about  10  cm.  above  a  low  Bunsen  burner 
flame  for  five  to  ten  minutes,  so  that  any  moisture  may  be  expelled. 
It  is  then  gently  lowered  until  the  bottom  is  a  faint  red,  and  is 
kept  thus  for  another  five  minutes  or  so.  The  flame  is  then  grad- 
ually raised  until  the  mass  is  in  a  state  of  quiet  fusion.  If  the 
operation  is  conducted  with  proper  care  and  slowness,  all  the  car- 
bon dioxide,  formed  in  the  reaction  between  the  silicates  and  car- 
bonate, can  be  driven  off  quietly  through  the  half-sintered  mass 
and  without  any  spattering. 

When  the  mass  is  in  fusion  the  height  of  the  flame  and  of  the 
crucible  above  it  are  so  adjusted  that  the  mass  is  liquid  and  of  a 
dull  red  (about  850°),  but  without  any  spattering  of  drops  onto  the 
cover.  The  crucible  is  to  be  kept  covered  during  the  operation, 
except  for  examination  of  the  contents.  This  will  demand  some 
attention  the  first  few  times  the  operation  is  done,  but  the  right 
conditions  are  soon  learned  with  practice  and  care.  The  melt  is 
to  be  kept  in  this  state  of  quiet  fusion  for  at  least  fifteen  minutes, 
during  which  slow  currents  can  be  observed  to  cross  it,  which 


134  METHODS 

resemble  those  in  a  quiet  lava-filled  crater,  such  as  that  of 
Kilauea. 

The  liquid  will,  with  some  rocks,  not  be  perfectly  clear  and 
transparent,  as  the  carbonates  of  iron,  calcium,  and  magnesium 
will  form  cloudy  masses  within  it;  so  that  any  such  appearance 
need  not  cause  concern.  Indeed,  with  rocks  that  are  very 
high  in  lime,  magnesia,  and  the  iron  oxides,  the  mass  may 
appear  to  be  only  half  fused,  because  of  the  abundance  of  these 
substances,  although  the  rock  is,  in  reality,  completely  decom- 
posed. Some  sodium  carbonate  will  usually  vaporize  and  con- 
dense on  the  under  side  of  the  cover,  but  this  is  of  no 
consequence. 

When  the  whole  operation  has  lasted  from  the  beginning  at 
least  one-half  to  three-quarters  of  an  hour,  and  it  is  judged  that 
decomposition  is  complete,  the  crucible  is  taken  from  the  flame 
and  is  placed  on  a  clean,  cool  flat  surface  of  iron  or  polished  stone. 
Such  methods  for  quick  cooling  as  using  a  blast  of  air,  or  dipping 
into  water,  are  never  to  be  used,  as  they  injure  the  crucible  and 
greatly  shorten  its  life.  They  also  make  impossible  a  neat  removal 
of  the  cake  from  the  crucible.  Hillebrand  recommends  giving  the 
crucible  a  quick  rotary  motion  before  placing  on  the  slab,  so  as  to 
spread  the  melt  over  the  sides  in  a  thin  sheet.  This  certainly  has 
the  advantage  of  rendering  the  subsequent  disintegration  more 
rapid,  and  also  to  some  extent  facilitates  the  separation  of  the 
cake  from  the  crucible.  It  is  not,  however,  necessary,  and  in 
general  I  am  content  to  cool  the  crucible  quickly  but  quietly  on  a 
slab  of  polished  granite. 

During  the  first  moments  of  cooling  the  melt  should  be  watched, 
and  if  it  bubbles  or  forms  miniature  craters,  this  may  be  taken  as 
evidence  that  the  decomposition  and  the  expulsion  of  C02  are 
not  complete.  In  this  case  the  whole  should  be  remelted  and  kept 
at  a  bright-red  heat  for  another  ten  minutes. 

It  is  very  important  that  the  crucible  and  its  contents  be 
thoroughly  cold  before  the  removal  of  the  cake  is  begun.  The 
contents  must  be  so  cold  that  they  separate  either  wholly  or  par- 
tially from  the  metal  walls.  If  water  is  poured  into  the  crucible 
before  the  cake  is  thoroughly  cold,  the  removal  of  the  cake  will 
probably  be  difficult.  It  is  always  better  to  be  patient  during  the 
cooling  process  and  to  allow  the  crucible  to  stand  more  time  than 


FUSION  WITH  SODIUM   CARBONATE  135 

may  be  actually  needed,  than  to  incur  the  possible  annoyance  of  a 
cake  that  obstinately  refuses  to  be  extricated. 

When  a  considerable  amount  of  pyrite  is  present  in  the  rock, 
it  is  necessary  to  oxidize  the  sulphur,  to  avoid  attacking  the 
crucible.  This  may  be  done  by  adding  a  very  little  potassium 
nitrate  to  the  carbonates.  But  even  a  small  quantity  of  this  gives 
rise  to  effervescence,  through  reaction  with  the  carbonates,  and 
hence  increases  the  possibility  of  loss  through  spattering.  There 
is  also  danger  of  attacking  the  crucible  through  the  action  of  the 
nitrate.  It  is,  therefore,  better  after  weighing  the  rock  powder 
and  before  the  addition  of  the  alkali  carbonate,  to  roast  the  rock 
powder  in  the  open  crucible  at  a  low  red  heat,  insufficient  to  sinter, 
and  far  less  to  fuse,  the  rock.  The  mass  can  then  be  mixed  with 
the  carbonates  and  the  fusion  proceeded  with,  as  described  above. 

As  a  general  rule  the  cold  cake  will  be  of  a  bluish-green  color, 
due  to  the  formation  of  sodium  manganate.  It  sometimes  hap- 
pens that  rocks  high  in  ferrous  oxide,  even  if  containing  consider- 
able manganese,  show  in  the  cooled  melt  not  a  trace  of  the  char- 
acteristic green,  but  only  a  muddy-brown  color,  due  to  dissem- 
inated ferric  compounds. 

Hillebrand  attributes  certain  irregularities  in  the  coloration  to 
the  presence  of  a  reducing  atmosphere  within  the  crucible,  under 
conditions  which  are  little  understood.  Thus  it  may  happen 
that  "  two  fusions  made  side  by  side  or  successively,  under  appa- 
rently similar  conditions,  may  in  one  case  show  little  or  no  man- 
ganese, in  the  other  considerable."  It  is  probable  that  all  rock 
analysts  have  had  similar  experiences. 

Removal  of  the  Cake. — Before  describing  the  removal  of  the 
cake  from  the  crucible,  one  or  two  points  in  regard  to  the  crucible 
itself  may  be  touched  on.  From  a  new  or  little-used  platinum 
crucible,  with  the  ordinary  amount  of  flare,  the  extraction  of  the 
cake  usually  offers  no  special  difficulties,  if  attention  be  paid  to  the 
small  points  mentioned  above  and  given  below.  But  after  a 
platinum  crucible  has  been  in  use  for  some  time,  especially  when  it 
is  often  heated  over  the  blast,  the  bottom  tends  to  drop,  and  so 
alters  the  shape  of  the  lower  part.  The  smooth,  single,  interior 
concave  curve  becomes  a  double,  ogee-like  one,  and,  being  slightly 
convex  inwardly,  frequently  gives  rise  to  difficulty  in  removing 
the  cake.  When  the  crucible  which  is  used  for  the  carbonate 


136  METHODS 

fusion  gets  into  this  condition,  it  is  well  to  return  it  to  the  maker 
and  have  it  re-formed. 

As  all  dents  and  other  irregularities  are  sure  to  cause  difficulty, 
the  platinum  crucible  should  never  be  allowed  to  fall  or  become 
dented.  Above  all,  any  squeezing  or  other  violent  pressure 
should  be  avoided  in  attempting  to  loosen  the  melt,  as  any  such 
deformations  will  greatly  decrease  the  usefulness  and  value  of  the 
crucible.  Caution  on  these  points  may  seem  superfluous,  but  one 
so  often  sees  battered  crucibles  in  use  in  laboratories,  especially 
in  the  hands  of  students,  that  the  reference  to  them  may  not  be 
amiss. 

The  thoroughly  cold  crucible  containing  the  cake  is  placed  in 
a  platinum  triangle  and  nearly  half  filled  with  water.  After 
standing  for  a  minute,  so  as  to  allow  the  water  to  creep  below 
the  cake,  it  is  gently  heated  over  a  small  flame.  The  flame  is 
cautiously  applied,  especially  around  the  edges  of  the  cake,  boiling 
being  avoided  as  likely  to  lead  to  loss.  After  the  edges  are  freed, 
the  bottom  is  gently  heated,  when,  under  favorable  circumstances, 
the  cake  loosens.  If  this  first  operation  is  not  successful,  the  fluid 
is  carefully  poured  out  into  the  platinum  basin,  any  drops  running 
over  the  edge  being  washed  into  the  basin  with  a  little  water  from 
the  wash-bottle.  The  crucible  is  then  again  half  filled  with 
water,  and  the  operation  repeated.  Two  or  three  repetitions 
will  usually  be  sufficient  to  attain  the  object.  An  undamaged, 
smooth  crucible,  patience,  and  gentle  heating  are  the  prime 
requisites  for  success  in  this  operation;  the  opposites  are  disas- 
trous. 

When  the  cake  is  loosened  it  is  transferred  to  the  platinum 
basin.  The  crucible  is  washed  slightly,  so  as  to  transfer  any  loose 
particles  to  the  basin.  Small  fragments  of  the  melt  may  still  adhere 
to  the  sides  of  the  crucible;  these  will  be  removed  by  subsequent 
treatment.  The  crucible  is  therefore  covered  and  set  aside.  The 
platinum  basin  (covered)  containing  the  cake,  and  not  more  than 
one-third  filled  with  water,  is  heated  on  the  water-bath,  or  over 
a  low  flame,  so  as  to  avoid  boiling,  until  the  cake  is  easily 
broken  up  with  the  spatula,  and  it  is  finally  more  or  less  dis- 
integrated. It  is  not  necessary,  nor  is  it  possible,  to  dissolve 
the  cake  entirely  in  the  water,  but  it  is  advantageous  that  it  be 
somewhat  disintegrated,  as  this  will  facilitate  the  solution  in 


FUSION  WITH  SODIUM   CARBONATE  137 

hydrochloric  acid.1  The  presence  of  a  few  small,  black  grains 
(of  magnetite  or  ilmenite)  need  not  cause  uneasiness,  as  they  are 
attacked  with  difficulty  by  the  carbonate,  but  will  be  dissolved 
by  the  acid. 

If  the  cake  should  prove  obstinate  and  refuse  to  loosen  from 
the  crucible,  one  of  two  plans  may  be  followed.  The  one  pre- 
ferred is  to  dissolve  the  cake  in  the  crucible  itself  over  a  low  flame 
or  on  the  water-bath.  The  liquid  in  the  platinum  basin  may  be 
used  for  this  operation,  in  small  portions  at  a  time,  the  crucible 
being  emptied  back  each  time.  The  other  consists  in  placing 
the  crucible  on  its  side  in  the  basin,  rilling  this  with  water  about 
one-third  full,  and  heating  gently  till  the  cake  is  dissolved.  The 
crucible  is  then  lifted  out  of  the  basin  by  means  of  a  stirring-rod, 
and  thoroughly  washed,  while  held  on  the  rod  above  the  basin, 
inside  and  out,  the  washings  falling,  of  course,  into  the  basin. 

Solution  of  the  Cake. — If  the  cake  is  green,  chlorine  will  be 
evolved,  on  the  addition  of  hydrochloric  acid,  through  reaction 
with  the  manganate,  and  will  attack  the  platinum.  To  avoid  this 
a  few  cubic  centimeters  of  alcohol  are  added  to  reduce  the  man- 
ganate. It  is  best  always  to  add  a  little  alcohol. 

When  the  cake  is  disintegrated,  the  platinum  spatula  is  removed 
and  washed  with  a  little  water  into  the  basin,  and  laid  aside  in  a 
clean  place.  The  basin  is  removed  from  the  flame  and  covered 
with  a  watch-glass,  which  should  project  about  one-half  an  inch  on 
all  sides.  This  is,  of  course,  placed  with  the  convex  side  down, 
as  must  always  be  done  with  covering  watch-glasses.  Fifteen 
or  20  c.c.  of  a  concentrated  hydrochloric  acid  are  measured  off  in  a 
25-c.c.  measuring-cylinder  with  lip,  and  poured  very  gradually 
into  the  basin  through  a  small  funnel,  the  end  of  which  has  been 
somewhat  drawn  out  and  bent  at  an  angle  of  45°,  so  as  to  project 
into  the  basin  through  the  lip-opening.  This  addition  of  acid 
should  be  very  gradual,  by  a  few  drops  at  a  time  at  first,  so  as  to 
allow  the  effervescence  to  be  as  gentle  as  possible.  It  is  also  well  to 
let  the  acid  flow  down  the  side  of  the  basin  below  the  lip,  so  that 
the  drops  thrown  up  by  the  first,  somewhat  violent,  effervescence 

1  While  this  disintegration  in  hot  water  is  generally  recommended,  it  is  not 
necessary,  and  time  will  usually  be  saved  by  decomposing  the  cake  directly 
with  hydrochloric  acid.  It  should  then  be  rubbed  occasionally  with  the  plat- 
inum spatula  so  as  to  facilitate  solution. 


138  METHODS 

may  be  directed  away  from  the  lip-opening.  A  pink  blush,  due 
to  MnCb,  indicates  the  presence  of  considerable  manganese. 

When  all  the  acid  has  been  added  except  1  or  2  c.c.  the  tip 
of  the  funnel  is  washed  into  the  crucible  with  a  little  water,  and 
the  funnel  is  withdrawn.  A  few  drops  of  acid  are  poured  on  the 
under  side  of  the  crucible  cover,  to  dissolve  any  drops  spattered 
from  the  fusion,  and  washed  into  the  crucible  with  a  very  little 
water.  The  rest  of  the  acid  is  then  poured  into  the  crucible,  to 
dissolve  any  adhering  portions  of  the  carbonate,  and  slightly 
warmed,  the  crucible  being  covered. 

The  basin  (covered)  is  heated  for  ten  minutes  or  so  on  the 
water-bath,  to  expedite  solution  in  the  acid,  and  to  drive  off  car- 
bon dioxide.  When  all  effervescence  has  ceased  in  the  basin,  this 
is  removed  from  the  water-bath,  the  drops  on  the  watch-glass 
cover  are  rinsed  down  into  it,  the  glass  being  held  vertically,  with 
the  part  which  has  been  next  the  lip  downward  and  near  the 
surface  of  the  liquid  in  the  basin.  The  rinsing  is  to  be  repeated 
several  times,  the  stream  being  so  directed  as  to  let  the  water  flow 
over  all  the  wetted  surface  from  top  to  bottom.  The  watch-glass 
is  laid  aside,  and  the  sides  of  the  basin  above  the  liquid  are  washed 
down  by  a  gentle  stream  from  the  wash-bottle,  the  basin  being 
slowly  revolved  to  facilitate  the  operation.  One  complete  washing 
down  all  around  will  be  sufficient.  The  contents  of  the  crucible 
are  then  added,  and  this  and  the  cover  rinsed  several  times  into  the 
basin.  When  the  operation  is  complete,  if  care  has  been  used  to 
avoid  an  inordinate  amount  of  wash- water,  the  basin  should  be 
little  more  than  half  full. 

As  a  little  silica  adheres  persistently  to  the  crucible  the  inside 
of  this  is  to  be  rubbed  with  a  small  piece  of  moist  filter  paper, 
which  is  then  thrown  into  the  basin.  This  is  disintegrated  during 
the  evaporation. 

The  platinum  spatula  is  then  put  in  the  basin,  resting  in  the 
lip,  and  this  placed  uncovered  on  the  water-bath  for  evaporation. 
The  fluid  should  be  clear,  and  contain  no  solid  except  possibly 
some  light  particles  of  silica.  There  may  be  a  few  small  black 
particles  of  magnetite  or  ilmenite  present,  which  will  dissolve  in 
the  hot  acid.  But  if  small,  hard,  gritty  particles  are  felt,  by  the 
spatula,  at  the  bottom,  the  fusion  has  not  been  successfully  carried 
out  to  complete  decomposition  of  the  rock,  and  the  contents 


SILICA  139 

of  the  basin  should  be  rejected,  another  portion  of  rock  powder 
weighed  out,  and  the  whole  operation  of  fusion  with  sodium  car- 
bonate gone  through  with  as  before.  This  should  never  happen, 
and  is  easily  avoided  by  sufficiently  long  fusion  with  the  sodium 
carbonate. 

6.  SILICA  J 

The  fluid  in  the  basin  now  contains  all  the  rock  constituents  in 
solution  as  chlorides,  except  the  silica,  which  is  for  the  most  part  in 
solution  as  a  soluble  silicic  acid,  and  partly  as  insoluble  particles. 
Our  first  object  then  is  to  separate  the  silica  from  the  other  con- 
stituents, so  that  it  may  be  weighed.  This  is  effected  by  evapora- 
tion to  dryness,  whereby  the  silica  is  rendered  insoluble  in  water. 

Errors. — It  has  been  shown  by  Hillebrand  and  others  that  a 
single  evaporation  does  not  render  all  the  silica  insoluble;  so  that 
two,  or  even  three,  evaporations  are  necessary  for  accurate  work. 
Prolonged  heating  at  110°-130°  is  apt  to  allow  some  of  the  silica 
to  be  dissolved  in  the  hydrochloric  acid  used,  as  well  as  to  increase 
the  impurities  in  the  silica.  This  is  due  to  recombination  of  the 
silica  with  some  of  the  bases,  mostly  either  with  magnesia  or  with 
soda. 

Complete  dehydration  of  the  silica  is  somewhat  uncertain  and 
is  apt  to  be  incomplete,  and  unless  the  silica  is  ignited  at  a  very 
high  temperature  it  is  apt  to  be  hygroscopic.  Blasting  for  twenty 
minutes  or  more  is  therefore  generally  recommended.  Although 
this  may  be  advisable,  it  may  cause  loss  in  weight  of  the  crucible, 
and  for  general  work  I  find  that  strong  ignition  over  a  good  Meker 
flame  for  twenty  minutes  gives  satisfactory  results.  This  is  prob- 
ably due  to  the  compensation  for  the  slightly  imperfect  dehy- 
dration by  the  small  amount  of  silica  that  goes  into  solution  and 
that  is  not  recovered  with  the  alumina. 

A  small  amount  of  silica  adheres  strongly  to  the  basin  and  will 
be  lost  if  it  is  not  rubbed  off.  It  would  appear  to  be  almost  impos- 
sible to  remove  this  silica  from  porcelain  with  a  "  policeman."  2 

1  Classen,  2,  pp.  605-611;  Fresenius,  1,  pp.  509-511;  Hillebrand,  pp.  91-97; 
Mellor,  pp.  166-167;   Treadwell,  2,  pp.  485-488;    Lenher  and  Truog,  Jour. 
Am.  Chem.  Soc.,  38,  p.  1059,  1916. 

2  Cf.  Hillebrand,  Jour.  Am.  Chem.  Soc.,  28,  p.  232,  1906. 


140  METHODS 

Practically  all  the  silica  that  is  not  rendered  insoluble  in  the 
main  evaporations  is  precipitated  with  the  alumina,  and  must  be 
recovered  from  the  solution  of  the  fusion  of  the  alumina  precipitate 
in  pyrosulphate,  though  a  little  of  it  is  lost  here,  as  silica  is  slightly 
soluble  in  this  reagent.  The  amount,  however,  is  always  small. 

The  weight  of  silica  must  always  be  corrected  for  the  impurities 
that  it  invariably  contains,  by  evaporation  with  hydrofluoric  acid. 

From  the  possible  errors  just  given  it  may  appear  that  the 
correct  determination  of  silica  is  so  fraught  with  difficulty  and 
uncertainty,  as  to  be  probably  unsatisfactory.  This  conclusion, 
however,  is  not  borne  out  by  the  facts.  In  the  first  place,  the  actual 
error  caused  by  each  of  these  possible  sources  of  error  is  generally 
very  small,  with  the  exception  of  that  due  to  impurity  in  the  silica 
which  can  be  eliminated,  however,  by  evaporation  with  hydrofluoric 
acid.  Secondly,  the  errors  apply  to  that  constituent  which  is 
present  in  by  far  the  largest  percentage  in  the  great  majority 
of  rocks,  and  so  they  are  of  comparatively  insignificant  influence. 
Indeed,  much  crictical  study  of  the  character  and  value  of  rock 
analyses  has  led  to  the  conclusion  that,  in  analyses  not  of  the  best 
quality,  the  figures  for  silica  (and  lime)  are  more  likely  to  be  nearly 
correct  than  those  of  the  other  constituents. 

Separation  of  Silica. — To  render  the  silica  insoluble,  the  solu- 
tion in  hydrochloric  acid  of  the  cake  from  the  sodium  carbonate 
fusion  is  evaporated  to  dryness  in  the  platinum  basin.1  This  is 
carried  out  on  the  water-  or  steam-bath  until  no  more  fumes  of 
hydrochloric  acid  are  given  off  and  the  mass  appears  to  be  quite 
dry,  the  dark-yellow  color  of  the  moist  salts  changing  to  a  pale- 
brown  shade.  During  the  last  stages  it  is  well  every  now  and 
then  to  break  up  the  gelatinous  mass  with  the  platinum  spatula, 
which  is  kept  in  the  basin,  so  that  the  water  and  hydrochloric 
acid  may  pass  off  more  readily.  When  the  mass  becomes  crys- 
talline, the  lumps  may  likewise  be  broken  up,  but  this  should  be 
done  with  caution  to  avoid  loss  by  flying  off  of  particles  of  the  salts. 

It  is  recommended  by  some  that  the  dry  salts  be  heated  for 
some  time  at  a  temperature  of  110°  or  120°.  This,  however,  is 
highly  disadvantageous  and  should  not  be  done,  as  apparently  sili- 

-  A  porcelain  basin  may  be  used,  but  it  is  less  satisfactory,  as  it  is  liable  to 
contaminate  the  liquid,  and  because  it  is  difficult  to  remove  the  silica  com- 
pletely from  the  porcelain  surface.  A  glass  basin  must  not  be  used. 


SILICA  141 

cates  are  apt  to  be  formed  that  are  soluble  in  hydrochloric  acid  and 
so  lead  to  loss  of  silica.  At  the  same  time  the  prolonged  heating 
will  probably  add  considerably  to  the  impurities  in  the  silica. 

After  half  an  hour's  further  heating  on  the  water-bath,  when 
the  mass  is  dry,1  the  basin  is  removed  from  the  water-bath  and  the 
contents  are  moistened  with  5-10  c.c.  of  concentrated  hydro- 
chloric acid,  to  dissolve  the  basic  salts  of  alumina,  iron,  and  mag- 
nesia that  are  formed  during  the  evaporation.  The  small  amount 
of  salts  on  the  spatula  are  also  moistened  with  the  acid.  The 
mass  of  salts  should  be  made  only  pasty  with  the  acid,  as  too  much 
will  tend  to  prolong  the  filtration,  and  silica  is  appreciably  soluble 
in  strong  hydrochloric  acid.  The  pasty  mass  is  mixed  thoroughly 
with  the  spatula,  some  of  it  being  rubbed  around  the  line  that 
marks  the  original  border  of  the  liquid,  where  a  zone  of  strongly 
adherent  silica  is  apt  to  be  formed. 

Water  is  now  added  from  the  wash-bottle,  the  stream  washing 
down  the  sides  of  the  basin.  About  15-20  c.c.  of  water  in  all 
should  be  added.  The  spatula  is  rinsed  off  into  the  basin,  and  is 
cleaned  with  a  bit  of  filter  paper  which  is  dropped  into  the 
basin,  because  a  little  silica  (only  visible  when  the  spatula  is  dry), 
adheres  to  it  persistently.  The  basin  should  not  be  more  than  one- 
third,  at  the  most,  full  of  liquid. 

A  glass  stirring-rod,  about  1  inch  longer  than  the  diameter  of 
the  basin,  is  placed  in  this,  and  the  contents  are  heated  on  the 
water-bath  or  over  a  low  flame,  with  constant  stirring,  until  the 
chlorides  are  entirely  dissolved  and  only  insoluble  silica  remains. 
This  is  indicated  by  the  absence  of  gritty  particles  of  salt  on 
"feeling"  with  the  rod. 

While  the  solution  of  the  chlorides  is  being  effected  at  a  gentle 
heat  the  filter  may  be  made  ready.  A  9-cm.  filter  and  a  6J-cm. 
funnel  are  used.  The  filtration  is  carried  out  as  described  on  p. 
118,  a  400-c.c.  beaker  being  used  to  catch  the  filtrate. 

When  all  the  liquid  and  silica  that  will  pass  readily  with  it 

1  Mellor  (p.  175,  note  3)  suggests  the  addition  of  alcohol  before  drying,  so 
as  to  hasten  the  process.  I  have  used  this  for  some  years  in  the  drying  of  the 
chlorides  in  the  alkali  determination  (p.  199)  with  good  results,  and  I  think 
that  it  would  aid  in  the  dehydration  of  the  silica  also.  A  few  cubic  centimeters 
may  be  added  when  the  crystalline  mass  is  almost  dry  and  the  drying  is  then 
continued  to  completion. 


142  .       METHODS 

have  been  brought  on  the  filter,  the  basin  is  gently  rinsed  with  a 
little  cold  water  from  the  wash-bottle,  the  silica  adhering  to  the 
sides  being  washed  down  to  the  bottom,  and  the  liquid  and  as 
much  of  the  silica  as  possible  are  poured  into  the  filter  as  before. 
When  the  filter  is  empty,  the  basin  is  held  in  the  left  hand,  above 
the  filter,  with  the  stirring-rod  across  it  and  resting  on  the  lip,  the 
end  of  the  rod  an  inch  or  so  beyond,  and  the  rod  kept  in  place 
by  the  tip  of  the  left  forefinger.  A  gentle  stream  of  water  is  then 
directed  against  the  now  upper  side  of  the  basin,  so  as  to  wash  the 
silica  into  the  filter,  and  at  the  same  time  rinse  the  basin.  When 
the  filter  is  nearly  full  the  liquid  is  allowed  to  empty  and  the 
operation  repeated  until  the  silica  is  washed  thoroughly,  and  all 
the  silica  brought  into  the  filter  as  far  as  possible  without  too  many 
rinsings.  The  stirring-rod  is  washed  off  into  the  beaker. 

The  water  used  in  washing  the  silica  should  be  cold,  that  is  at 
room  temperature,  or  better  containing  a  little  hydrochloric  acid. 
This  is  because  hot  solutions  of  alumina  and  iron,  unless  decidedly 
acid,  hydrolyze  readily  and  throw  down  basic  salts  that  contam- 
inate the  silica.  It  may  happen  that  the  silica  becomes  brick- 
red  from  the  iron  present  if  it  is  washed  with  hot  water  that  con- 
tains no  acid 

Lenher  and  Truog  1  recommend  using  hot  water  containing 
5  per  cent  (by  volume)  of  hydrochloric  acid.  This  has  the  advan- 
tages of  diminishing  the  time  and  volume  of  liquid  needed  for 
washing,  helping  to  remove  the  bases  and  basic  salts  present  in  the 
silica,  and  also  preventing  the  silica  from  going  into  a  colloidal 
condition  and  so  passing  through  the  filter.  Their  suggestion 
may  well  be  adopted.  In  this  case,  pure  hot  water  from  the  wash- 
bottle  may  be  used,  and  a  little  hydrochloric  acid  added  to  the 
contents  of  the  filter  with  each  addition  of  the  wash  water. 

When  the  liquid  has  ceased  dropping  from  the  last  rinsing, 
the  platinum  basin  is  substituted  for  the  beaker  beneath  the  suc- 
tion-tube, taking  care  to  lose  no  drops  from  the  latter  during  the 
change.  The  contents  of  the  beaker  are  poured  into  the  basin, 
and  the  beaker  itself  is  rinsed  once  or  twice,  not  more,  the  rinsings 
going  also  into  the  basin.  The  beaker  and  basin  are  then  inter- 
changed once  more,  and  the  stirring-rod  is  placed  in  the  beaker 

1  Lenher  and  Truog,  Jour.  Am.  Chem.  Soc.,  38,  p.  1058,  1916.  Hillebrand, 
(p.  92)  does  not  recommend  this. 


SILICA  143 

set  beneath  the  funnel.  The  basin  with  the  platinum  spatula 
in  it,  is  once  more  placed  on  the  water-bath  for  the  second  evap- 
oration. 

While  the  second  evaporation  is  going  on  the  washing  of  the 
main  portion  of  silica  with  hot,  slightly  acid  water  is  completed. 
The  filtrate  is  received  in  the  400-c.c.  beaker  previously  used. 
The  portions  of  wash  water  added  each  time  should  be  small,  so 
as  to  keep  the  volume  of  filtrate  down. 

When  the  second  evaporation  is  complete  and  the  salts  are 
reduced  to  dryness  and  free  from  HC1,  occasional  stirring  with 
the  spatula  hastening  the  process,  the  mass  is  again  moistened 
with  a  little  (3-5  c.c.)  hydrochloric  acid,  and,  after  standing 
(warm)  five  minutes,  about  50  c.c.  of  water  are  added,  and  the 
whole  gently  heated  to  complete  solution  (except  for  particles  of 
silica). 

The  liquid  is  then  filtered  through  a  separate  7-cm.  filter,  the 
basin  is  well  rinsed,  and  the  filtrate  and  washings  caught  in  the 
400-c.c.  beaker  containing  the  previous  filtrate.  The  small 
amount  of  silica  is  to  be  brought  into  the  filter,  and  the  basin 
rubbed  with  a  small  piece  of  moist  filter  paper  to  remove  adherent 
silica.  There  will  be  need  of  but  little  washing  for  this  portion. 
Hot  water  containing  about  5  per  cent  of  hydrochloric  acid  may 
be  used. 

When  washing  is  complete  the  bulk  of  liquid,  including  all 
the  washings,  in  the  400-c.c.  beaker  should  not  be  more  than 
200  c.c.,  if  the  operation  has  been  conducted  with  care  and  due 
avoidance  of  excessive  use  of  liquid. 

Ignition  of  Silica. — A  platinum  crucible  of  25  or  35  c.c. 
capacity  is  selected,  preferably  the  latter  if  the  rock  contains 
much  alumina  or  iron,  ignited,  cooled  in  the  desiccator  and  weighed. 
The  second  filter,  that  contains  the  extra  silica  from  the  second 
evaporation,  is  removed  from  the  funnel  with  the  platinum  spatula, 
placed  in  the  crucible,  heated  until  the  paper  is  carbonized,  and 
then  reduced  to  ash.  This  will  take  but  ten  minutes  or  so. 

When  the  crucible  is  cold,  the  free  edges  of  the  first  filter,  that 
contains  the  main  portion  of  silica,  are  folded  down  on  the  silica 
so  as  to  enclose  this  completely,  the  platinum  spatula  being  used 
for  this.  The  little  package  is  then  transferred  with  the  spatula 
to  the  crucible,  placed  above  the  ashes  in  it,  and  preferably  with 


144  METHODS 

the  side  uppermost  that  has  three  thicknesses  of  paper.  The 
package  is  very  gently  worked  and  pressed  down  toward  the  bot- 
tom of  the  crucible,  but  the  paper  should  not  be  torn,  nor  should  all 
egress  for  steam  from  below  be  shut  off.  With  a  small  piece  of 
filter  paper  any  particles  of  silica  adhering  to  the  spatula  are  rubbed 
off,  and  also  any  which  may  be  on  the  funnel  above  the  edge  of  the 
filter,  and  the  piece  of  paper  is  also  placed  in  the  crucible.  In 
this  way  the  silica  can  be  dried  in  the  crucible  and  ignited,  with  no 
danger  of  loss  from  whirling  up  of  the  light  powder. 

The  covered  crucible  (vertical)  is  first  heated  at  about  15  cm. 
above  a  low  flame,  the  heating  being  cautious  so  as  to  avoid  boiling 
of  the  pasty  mass,  and  probable  loss  of  substance  or  spattering  of 
it  on  the  sides  of  the  crucible.  This  is  continued  till  the  contents 
are  dry  and  the  filter  begins  to  char.  As  the  water  is  driven  off 
the  crucible  is  gradually  lowered,  but  this  must  be  done  with  great 
caution,  and  the  flame  kept  small.  A  filter  which  is  carbonized 
at  a  low  temperature  is  more  easily  incinerated  than  one  which  is 
carbonized  rapidly  and  at  a  high  temperature.  The  crucible  is 
finally  brought  close  to  the  flame  and  heated  till  no  more  smoke  is 
given  off.  The  escaping  vapors  should  never  be  allowed  to  ignite, 
and  consequently  the  flame  should  be  kept  low  and  the  bottom  of 
the  crucible  should  not  be  brought  to  a  red  heat  till  carbonization 
is  complete. 

The  almost  full  flame  is  then  turned  on  and  the  crucible  heated 
to  a  bright-red  heat,  being  kept  vertical  and  with  the  cover  very 
slightly  moved  to  one  side,  so  as  to  allow  the  entrance  of  some  air, 
but  not  enough  to  give  rise  to  dangerous  draughts.  The  flame,  of 
course,  should  not  be  allowed  to  envelop  the  crucible,  as  an  oxidiz- 
ing atmosphere  within  it  is  essential.  When  the  carbon  is  entirely 
consumed,  or  almost  so,  the  cover  is  put  in  place,  a  Meker  sub- 
stituted for  the  Bunsen  burner,  and  the  crucible  is  heated  to  a 
bright  red  for  at  least  thirty  minutes.  This  is  necessary  in  order 
to  effect  complete  dehydration  of  the  silica,  the  last  portions  of 
water  being  retained  with  great  tenacity.  It  also  has  the  advan- 
tage of  rendering  the  silica  non-hygroscopic.  It  is  well  to  reheat 
the  crucible  once  or  twice  to  constant  weight.  The  cover  should 
be  examined  to  see  if  it  carries  any  adhering  carbon,  and  if  so  this 
is  to  be  burnt  off  by  heating  in  the  flame. 

The  crucible  and  its  contents  are  then  cooled  in  the  desiccator 


SILICA  145 

i 

and  weighed.  The  result  is  to  be  recorded  as  Cruc.+Si02-hx, 
above  the  weight  of  the  empty  crucible,  and  also  on  the  same  line 
to  the  right  of  it. 

The  silica  as  thus  obtained  is  never  pure,  but  contains  small 
amounts  of  AbOs,  Fe2Os,  TiCb,  P2O5,  and  possibly  other  impuri- 
ties. In  "  basic  "  rocks  these  may  amount  to  one-half  of  one  per 
cent  or  more.  The  correction  of  silica  for  these  must  not  be 
omitted  on  any  account,  no  matter  what  may  be  the  kind  of  rock 
or  silicate. 

After  weighing,  therefore,  the  silica  is  moistened  with  1  or  2  c.c. 
of  water.  In  doing  this  the  tip  of  the  wash-bottle  should  be  filled 
with  water  by  blowing  before  inserting  in  the  crucible,  to  avoid 
blowing  out  any  of  the  light  silica  by  the  first  puff  of  air  from  the 
empty  tip.  The  stream  is  directed  against  the  side  of  the  crucible, 
the  tip  being  inserted  below  the  slightly  raised  cover. 

Three  or  four  drops  of  dilute  sulphuric  acid  are  then  added, 
this  being  necessary  to  retain  the  Ti(>2,  some  of  which  would  be 
vaporized  as  titanium  fluoride  in  the  absence  of  sulphuric  acid. 
Hydrofluoric  acid  is  then  poured  in,  a  few  drops  at  a  time.  The 
action  is  apt  to  be  violent,  but  with  care  and  sufficient  moistening 
of  the  silica  no  loss  need  be  incurred.  The  hydrofluoric  acid 
should  be  added  in  quantity  sufficient  to  dissolve  all  the  silica  on 
warming.  Five,  or  at  most  10  c.c.  should  be  ample  for  this  pur- 
pose. 

The  crucible  is  then  placed  in  the  triangle  of  a  radiator,  such 
as  is  described  and  figured  by  Hillebrand.1  If  this  is  not  available, 
a  capacious  (50-  or  60-c.c.)  nickel 2  crucible,  with  an  appropriate 
triangle  made  of  nickel  or  platinum  wire,  serves  admirably.  The 
triangle  is  bent  or  spread  out,  so  that  the  platinum  crucible  is  well 
down  in  the  radiator,  but  with  the  sides  not  touching.  This 
arrangement  assures  uniform  heating  of  the  liquid  and,  with  care, 
prevents  "  creeping  "  or  loss  by  spattering.  The  radiator  and 
crucible  within  it  are  heated  over  a  moderately  low  flame,  so  that 
the  liquid  never  boils,  until  the  contents  are  nearly  dry.  Toward 
the  end,  when  only  a  few  drops  of  sulphuric  acid  are  left,  the  oper- 
ation can  be  hastened  by  cautiously  heating  the  bottom  of  the 

1HiUebrand,  p.  31. 

2  A  porcelain  crucible  may  be  used  if  one  of  nickel  is  not  at  hand,  but  the 
evaporation  is  not  so  rapid. 


146  METHODS 

naked  platinum  crucible  with  a  small  flame  waved  beneath  it 
until  all  the  acid  is  driven  off.  The  whole  of  this  operation  must 
be  conducted  under  a  hood  that  is  provided  with  a  good  draught. 
The  crucible  is  then  ignited  at  a  bright-red  heat,  blasting 
for  a  few  minutes  being  advisable  to  ensure  the  decomposition 
of  the  sulphates  of  iron  and  titanium,  and  the  complete  expulsion 
of  all  traces  of  sulphuric  acid.  After  cooling  in  the  desiccator 
the  crucible  is  weighed,  and  its  weight  noted  as  Cruc.+x  below 
that  of  Cruc.+Si02+#.  To  this  weight  of  silica  is  to  be  added 
later  that  of  the  small  portion  that  is  recovered  from  the  pyrosul- 
phate  fusion  (p.  162),  before  the  percentage  of  silica  can  be  cor- 
rectly calculated. 

As  pointed  out  by  Hillebrand,  the  composition  of  this  residue 
is  variable.  It  always  contains  alumina,  and  ferric,  titanic  and 
phosphoric  oxides,  very  exceptionally  baryta,  but  no  lime  or  mag- 
nesia if  the  rock  has  been  properly  decomposed.  The  assumption 
that  all  the  titanium  is  present  in  this,  or  that  the  residue  consists 
almost  only  of  Ti02,  is  quite  unwarranted. 

.  The  crucible  containing  the  impurities  in  the  silica  may  be  laid 
aside  in  a  desiccator  or  other  safe  place,  undeaned,  for  use  in  the 
subsequent  ignition  of  the  precipitate  of  alumina,  etc.  (p.  157). 

7.  ALUMINA  PRECIPITATE 

The  filtrate  from  the  silica  contains,  of  the  main  constitu- 
ents, the  aluminum,  iron,  titanium,  manganese,  zirconium,  and 
phosphorus  oxides,  which  are  conjointly  separated  from  the  lime, 
magnesia,  and  alkalies  also  present,  by  precipitation  with  ammonia 
water.  An  alternate  method  is  precipitation  with  sodium  acetate, 
but,  for  general  purposes,  this  is  much  inferior  to  the  other.  The 
ammonia  precipitation  will,  therefore,  be  described  first,  and  a  brief 
description  of  the  "  basic  acetate  "  method  will  follow. 

It  should  also  be  noted  that  this  filtrate  contains  some  plat- 
inum.1 Some  of  this  comes  from  the  crucible  in  which  the  sodium 
carbonate  fusion  has  been  made.  Sodium  carbonate  loses  a  little 
carbon  dioxide  on  heating,  even  at  800°,  and  experiments  by 
E.  G.  Zies  in  the  Geophysical  Laboratory  indicate  that  it  is  the 
sodium  oxide  so  formed  that  attacks  the  platinum.  The  greater 

!.Cf.  Hillebrand,  p.  97;  Treadwell,  pp.  110,  493. 


ALUMINA  PRECIPITATE  147 

part  comes  from  the  platinum  basin  in  which  the  evaporation  to 
dryness  has  been  carried  out.  Ferric  chloride  is  reduced  to  ferrous 
by  digestion  with  platinum,  which  forms  chloroplatinic  acid. 
This  action  will  be  the  greater  the  more  iron  the  rock  contains. 
It  is  not  necessary  to  remove  this  platinum  before  precipitation 
with  either  ammonia  or  sodium  acetate. 

Errors  in  Alumina. — Of  all  the  determinations  of  chemical 
constituents  of  rocks,  that  of  alumina  is  the  most  liable  to  error, 
not  only  in  magnitude  but  in  variety.  This  is  due  in  part  to  its 
amphoteric  character;1  in  part  to  the  gel-like  consistence  of  its 
hydroxide,  and  consequent  tendency  to  adsorption  of  salts  and 
difficulty  in  washing,  as  well  as  its  liability  to  pass  through  the 
filter;  in  part  to  the  tendency  of  magnesia  to  be  precipitated  with 
it;  in  large  part  to  the  present  necessity  of  determining  alumina 
by  difference  (as  no  method  is  yet  known  for  the  satisfactory  com- 
plete separation  of  alumina  from  all  other  constituents  or  for  its 
direct  determination),  so  that  errors  committed  elsewhere  fall 
on  it;  and  in  part  to  other  causes. 

The  correct  determination  of  alumina  is,  undoubtedly  more 
troublesome  and  difficult  than  that  of  any  of  the  other  constituents, 
and  errors  incidental  to  it  have  caused  the  rejection  of  many  other- 
wise fairly  good  analyses.2 

Especial  care  must,  therefore,  be  taken  in  all  the  manipulations 
and  precautions  that  are  involved  in  the  determination  of  the 
various  other  constituents  that  are  weighed  with  it.  On  the 
whole,  the  tendency  is  toward  a  plus  error  in  its  determination. 

Although  the  remarks  above  and  the  following  list  of  errors 
may  appear  formidable  and  tend  to  discouragement,  yet,  as  a 
matter  of  fact,  they  should  not  be  so  regarded.  To  the  careless 
or  slovenly  analyst  the  determination  of  alumina  is  indeed  beset 
with  pitfalls;  but  if  the  proper  precautions  are  observed  the  de- 
termination of  alumina  may  be  carried  out  with  almost  as  much 
accuracy  as  that  of  the  other  constituents,  in  spite  of  its  in- 
evitable tediousness. 

If  ammonium  salts  are  not  present  in  sufficient  amount  some 
magnesia  is  precipitated  with  the  alumina,3  thus  increasing  the 

1  Cf .  Stieglitz,  1,  pp.  171  ff. 

2  Cf.  H.  S.  Washington,  Prof.  Paper  99,  pp.  14,  17;  also  Hillebrand,  p.  98. 
3Cf.  Ostwald,  p.  150;  Stieglitz,  1,  pp.  168,  170,  191. 


148  METHODS 

apparent  araount  of  the  alumina  and  diminishing  by  just  as  much 
that  "of  the  magnesia.  This  is  a  very  frequent  error  especially 
among  the  earlier  analyses,  and  is  but  too  often  committed  at  the 
present  day.  It  is  especially  liable  to  occur  in  rocks  that  are  high 
in  magnesia,  and  should  be  carefully  guarded  against  by  the 
analyst;  the  more  so,  as  this  error  is  very  easy  to  prevent.  The 
analyst  must,  therefore,  be  sure  that  there  is  an  abundance, 
(even  a  superabundance),  of  ammonium  salts,  either  the  chloride 
or  the  nitrate,  in  the  liquid.  The  presence  of  such  salts  prevents 
the  formation  of  colloidal  solutions  and  the  consequent  "running 
through  "  the  filter  of  the  aluminum  and  iron  hydroxides. 

The  first  precipitate,  whether  by  ammonia  or  sodium  acetate, 
should  always  be  dissolved  and  reprecipitated,  even  if  the  rock 
contains  but  little  magnesia.  This  double  precipitation  is  neces- 
sary because  of  the  tendency  shown  by  the  gelatinous  precipitate 
to  adsorb  dissolved  salts,  so  that  the  first  precipitate  is  invariably 
contaminated  with  salts  of  lime,  magnesia,  and  the  alkalies.  If 
the  rock  is  high  in  magnesia,  a  second,  or  even  a  third,  reprecipita- 
tion  may  be  necessary  to  remove  these. 

If  the  ammonia  water  used  is  not  fresh  and  contains  ammonium 
carbonate,  some  calcium  carbonate  will  be  thrown  down  with  the 
alumina,  and  will,  of  course,  increase  the  apparent  amount  of 
alumina  and  diminish  that  of  the  lime  to  the  same  extent.  The 
ammonia  water  should,  therefore,  before  using,  be  tested  with 
calcium  or  barium  chloride,  and  if  a  precipitate  forms  the  ammo- 
nia water  should  be  rejected  or  redistilled  from  slaked  lime.1 

If  ammonia  water  is  kept  in  a  glass  bottle,  this  is  sure  to  be 
acted  on  by  the  alkaline  liquid,  rendering  the  ammonia  water 
impure,  sometimes  even  to  the  extent  of  showing  flakes  of  silica 
or  partially  decomposed  glass.  Such  ammonia  water  is  not 
uncommon  in  many  laboratories,  but  should  be  unhesitatingly 
rejected  as  totally  unfit  for  use,  even  after  filtration.  For  use 
in  good  work  ammonia  water  should  never  be  kept  in  glass,  unless 
this  is  coated  with  ceresine.  Even  here  the  ammonia  is  apt  to 
work  its  way  beneath  the  wax  and  become  impure. 

It  is  best  made  by  passing  the  gas  into  ice-cold  water  contained 
in  a  ceresine  bottle  (p.  48),  in  which  it  should  be  kept.     If  bought 
in  glass  it  should  be  transferred  to  ceresine  as  soon  as  possible. 
1  Cf.  Treadwell,  2,  p.  149. 


ALUMINA  PRECIPITATE  149 

In  very  accurate  work  the  ammonia  precipitation  should  be  car- 
ried out  in  vessels  of  platinum  or  gold. 

If  crystalline  salts  are  not  present  the  hydroxides  of  aluminum 
and  iron  tend  to  form  colloidal  solutions  and  pass  through  the 
filter.  This  tendency  is  less  noticeable  during  the  transfer  of  the 
precipitate  to  the  filter  than  during  the  washing  later.  It  can  be 
prevented  by  adding  ammonium  chloride  or  nitrate  to  the  wash 
water,  and  having  this  hot. 

Prolonged  boiling  or  standing  after  the  addition  of  ammonia 
is  to  be  avoided,  as  it  tends  to  make  the  precipitate  slimy  and  hard 
to  filter,  and  gives  more  opportunity  for  precipitation  of  lime  by 
the  atmospheric  carbon  dioxide.  On  prolonged  boiling,  further- 
more, the  ammonium  chloride  present  may  dissociate,  leading  to 
re-solution  of  some  alumina  by  the  hydrochloric  acid  set  free, 
unless  the  liquid  is  sufficiently  ammoniacal.  On  the  other  hand, 
ammonia  in  large  excess  may,  and  probably  will,  dissolve  some 
aluminum  hydroxide,  which  will  come  down  with  the  lime.1 

It  was  formerly  2  recommended  that  the  precipitate  be  washed 
entirely  free  from  chlorides,  because  of  the  possibility  of  loss  of 
aluminum  and  ferric  chlorides  by  volatilization.  As  regards 
aluminum,  however,  it  has  been  shown  3  that  there  is  no  such  loss, 
nor,  according  to  Daudt,4  is  there  such  loss  of  iron  if  the  amount 
of  ammonium  chloride  in  the  precipitate  does  not  exceed  about  1 
per  cent,  which,  in  analytical  practice  it  never  should. 

If  the  basic  acetate  method  is  used  for  the  first  precipitation 
there  is  a  strong  probability  that  some  of  the  alumina  and  ferric 
oxide  will  not  be  precipitated,  and  will  pass  through  the  filter. 
This  can  be  avoided  if  the  conditions  as  to  the  amount  of  free 
acetic  acid  are  very  accurately  adjusted,  so  that  care  and  strict 
attention  should  be  paid  to  the  suggestions  made  in  the  descrip- 
tion of  the  method. 

But  even  under  favorable  circumstances,  and  in  the  hands  of 
experienced  analysts,  a  little  alumina  is  liable  to  be  found  in  the 
filtrate,  particularly  with  rocks  high  in  alumina  and  low  in  iron. 

1  Cf.  Stieglitz,  1,  p.  196. 

2  Second  edition,  p.  101. 

3  Hillebrand,  Bull.  422,  p.  99,  note  c;  W.  Blum,  Jour.  Am.  Chem.  Soc.,  38, 
p.  1294,  1916. 

4  H.  W.  Daudt,  Jour.  Ind.  Eng.  Chem.,  7,  p.  847,  1915. 


150  METHODS 

This  should  always  be  recovered  before  precipitation  of  the  man- 
ganous  oxide,  though  this  precaution  is  frequently  neglected, 
apparently  through  ignorance  of  the  necessity  for  it.  The  magni- 
tude of  error  is  usually  not  very  great,  but  may  reach  as  much  as 
2  per  cent  of  the  rock,  judging  from  some  analysis  with  such  abnor- 
mally, and  otherwise  inexplicably,  high  percentages  of  manganous 
oxide.1 

The  basic  acetate  method  should,  for  these  reasons,  not  be 
used  by  the  inexperienced  analyst,  and,  unless  the  use  of  it  is  made 
necessary  by  the  presence  of  much  manganous  oxide,  it  is  best 
avoided  altogether  in  rock  analysis,  and  in  any  case  it  should  be 
carried  out  with  the  greatest  care.  It  may  be  noted  that  nearly 
all  the  authorities2  emphasize  the  difficulties  of  the  method  and 
even  advise  against  its  use. 

A  further  source  of  error  affecting  the  alumina  determination  is 
incomplete  reduction  of  ferric  to  ferrous  iron  for  the  determination 
of  the  total  iron  oxides  (p.  162).  The  unreduced  ferric  oxide  will 
not  affect  the  permanganate  and  hence  will  appear  as  alumina. 
The  method  of  separation  of  alumina  from  iron,  by  fusion  of  the 
ignited  precipitate  with  sodium  hydroxide  in  a  silver  crucible, 
which  is  sometimes  recommended,  should  never  be  used.  It  is 
open  to  grave  objections  and  offers  no  sufficiently  compensating 
advantages. 

Precipitation  by  Ammonium.3 — To  the  filtrate  from  the  silica 
in  the  400-c.c.  beaker,  which  should  amount  to  from  150  to  200 
c.c.  in  bulk,  10  c.c.  of  concentrated  hydrochloric  acid  are  added.4 
The  object  of  this  is  to  form  ammonium  chloride  on  the  addition 
of  ammonia,  in  sufficient  quantity  to  prevent  the  precipitation  of 
magnesia  along  with  the  alumina  and  iron.  One  should  also  avoid 

1  Cf.  H.  S.  Washington.  Prof.  Paper  99,  pp.  17,  20,  21;  1917. 

2  Fresenius,  1,  p.  647;  Classen,  1,  p.  465;  Hillebrand,  Bull.  422,  pp.  100, 113, 
116;  Jannasch,  p.  319;  Mellor,  pp.  177,  362;  Treadwell,  2,  p.  153. 

3  Classen,  1,  p.  562;   Fresenius,  1,  pp.  623-625;   Hillebrand,  pp.  98-103; 
Mellor,  pp.  177-183;   Treadwell,  2,  pp.  493-494;   H.  W.  Daudt,  Jour.  Ind. 
Eng.  Chem.,  7,  p.  848;  W.  Blum,  Jour.  Am.  Chem.  Soc.,  38,  pp.  1282-1297. 

4  Addition  of  nitric  acid  is  not  necessary,  as  the  ferrous  oxide  will  have 
been  changed  to  ferric  in  the  fusion  and  the  evaporation.     It  will  be  safer, 
however,  to  add  a  few  drops.     Nor  is  separation  of  platinum  necessary,  as 
recommended  by  Treadwell;    on  the  contrary  its  separation  at  this  stage 
would  be  very  disadvantageous. 


ALUMINA  PRECIPITATE  151 

too  large  an  excess  of  ammonium  chloride,  so  that  for  rocks  like 
granites  and  trachytes,  which  contain  but  little  magnesia,  the 
addition  of  5  c.c.  of  hydrochloric  acid  will  be  sufficient.  If  the 
rock  is  very  rich  in  magnesia  15  c.c.  will  not  be  too  much. 

At  this  point  the  analyst  has  to  decide  whether  or  not  to 
precipitate  the  manganese  with  the  alumina.  For  accurate  work 
my  preference  is  for  the  coprecipitation,  especially  if  the  rock  is 
rather  low  in  silica  and  the  color  of  the  carbonate  melt  shows  that 
much  manganese  is  present,  because  it  thus  affects  the  alumina 
alone  and  can  be  corrected  for  by  a  separate  determination.  In  this 
case  about  one-half  gram  of  solid  ammonium  persulphate,  which 
has  been  specially  purified,  is  added  to  the  liquid  in  the  beaker. 
The  manganese  will  be  thrown  down  as  peroxide,  but  nickel  and 
chromium  will  remain  in  solution,  the  former  as  an  ammonium 
double  salt,  and  the  latter  as  chromate.  Cobalt  will  be  precip- 
itated, but  as  no  more  than  traces  of  this  are  ever  present,  this  is 
of  no  moment.  Hillebrand  1  points  out  that  if  the  rock  carries 
appreciable  quantities  of  barium  or  strontium,  or  is  very  rich  in 
lime,  the  use  of  the  persulphate  is  not  advisable.  But  two,  and 
certainly  three,  precipitations  will  surely  get  all  the  lime  into  the 
filtrate  in  any  igneous  rocks,  any  barium  which  may  come  down 
can  be  collected  and  allowed  for  later,  and  it  is  only  in  very  excep- 
tional cases  that  the  amount  of  strontium  is  appreciable  here.  If 
the  rock  is  high  in  silica  or  is  low  in  manganese  (less  than  0.20 
per  cent),  as  is  true  of  nearly  all  rocks,  the  analyst  may  advan- 
tageously dispense  with  the  addition  of  the  persulphate,  and  dis- 
regard the  slight  error  involved  in  the  distribution  of  the  man- 
ganese among  the  alumina,  lime  and  magnesia. 

After  the  addition  of  the  hydrochloric  acid  (and  possibly  per- 
sulphate), a  few  drops  of  methyl  orange  are  added,2  so  as  to  be 
able  to  control  the  amount  of  ammonia  water  added.  The  liquid 
is  then  heated  nearly  to  boiling,  when  about  50  c.c.  of  ammonia 
water  are  poured  into  a  100-c.c.  beaker  3  and  diluted  with  half  as 

1  Hillebrand,  p.  102. 

2  Blum  (op.  cit.,  p.  1288)  recommends  either  methyl  red  or  rosolic  acid. 
His  experiments  show  that  these  are  better,  but  for  the  usual  practice  the 
commonly  obtainable  methyl  orange  will  serve.     If  nitric  acid  is  used,  it  must 
be  remembered  that  this  decolorizes  methyl  orange  or  methyl  red. 

3  This  is  done  to  guard  against  any  particles  of  ceresine  finding  their  way 
into  the  liquid.     Any  present  are  to  be  removed. 


152  METHODS 

much  water.  This  is  then  poured  very  slowly  and  cautiously, 
and  with  constant  stirring,  into  the  hot  liquid.  The  beaker  con- 
taining this  may  be  left  on  the  wire  gauze,  but  with  the  flame 
removed.  Enough  ammonia  is  added  to  give  a  slight  odor  and  the 
liquid  allowed  to  settle  a  bit.  If  the  clear  liquid  is  still  red  (acid) 
a  little  more  ammonia  is  added  until  it  just  turns  yellow.  If  it  is 
yellow,  it  is  best  to  add  hydrochloric  acid,  drop  by  drop,  to  acidity, 
and  then  very  slightly  (2-3  c.c.)  more  than  neutralize  with  ammo- 
nia water.  This  careful  procedure,  the  principle  of  which  was 
suggested  by  Blum,  insures  the  complete  precipitation  of  the 
alumina  and  the  other  constituents  with  it,  and  at  the  same 
time  it  prevents  the  solution  of  alumina  by  excess  ammonia. 

The  liquid  is  then  boiled  for  not  more,  and  better  less,  than  one 
minute  l  and  allowed  to  cool  until  the  beaker  can  be  handled. 
It  is  then  filtered,  still  hot,  through  an  11-cm.  filter  placed  in  a 
7.5-cm.  funnel,  the  filtrate  being  received  in  an  800-c.c.  beaker. 
This  size  of  filter  is  appropriate  for  most  rocks,  but  if  the  amount 
of  Al2O3+Fe20s  is  much  more  than  30  per  cent  it  is  well  to  filter 
through  two  9-cm.  filters  simultaneously,  with  a  600-c.c.  beaker 
beneath  each.  The  former  procedure  will  be  assumed  in  what 
follows.  It  is  not  necessary  to  take  special  precautions  against 
the  precipitate  passing  into  the  filter,  though  the  greater  bulk 
of  the  liquid  can  be  generally  transferred  with  but  little  of  the  pre- 
cipitate. Several  washings  by  decantation,  as  are  recommended 
by  some,  are  not  only  unnecessary  but  disadvantageous,  as  they 
will  add  much  to  the  volume  of  the  filtrate. 

The  beaker  is  rinsed  out  only  two  or  three  times  with  hot 
water,  which  also  serves  to  wash  the  precipitate  in  the  filter. 
This  is  not  to  be  washed  clean  at  this  stage.  To  prevent  the  forma- 
tion of  a  colloidal  solution,  a  cubic  centimeter  or  so  of  dilute  solu- 
tion of  ammonium  chloride  should  be  added  to  the  filter  with  each 
addition  of  wash  water.  The  beaker  is  not  to  be  cleaned. 

After  rinsing  the  beaker,  the  precipitate  in  the  filter  is  washed 
several  times  with  hot  water,  the  stream  from  the  wash-bottle 
breaking  it  up  more  or  less.  In  this  operation  great  care  should  be 
taken  not  to  throw  too  hard  or  sudden  a  jet  onto  the  precipitate, 

1  Boiling  for  longer  than  this  is  apt  to  give  rise  to  the  troubles  mentioned 
on  p.  149,  and,  besides,  is  quite  unnecessary.  The  liquid  is  apt  to  bump 
badly. 


ALUMINA  PRECIPITATE  153 

which  might  easily  throw  some  of  it  out  of  the  funnel.  Complete 
washing  is  not  necessary  at  this  stage,  but  the  precipitate  should 
be  collected  in  the  bottom  of  the  filter,  and  the  upper  edges  washed 
clean. 

In  filtering  and  washing  the  ammonia  precipitate  it  is  of  the 
highest  importance  that  the  mass  be  not  allowed  to  become 
dry.  Not  only  will  cracks  form  that  permit  the  wash  water  to 
run  through  without  removing  soluble  salts,  but  the  mass  will 
become  hard  and  difficult  or  impossible  to  break  up  without  danger 
of  loss.  The  transference  of  the  liquid  and  precipitate  to  the 
filter  and  the  washing  should,  on  this  account,  not  be  interrupted. 
This  is  the  filtration,  above  all  the  others,  to  which  attention  must 
be  paid  throughout,  and  the  analyst  will  soon  realize  the  gain  in 
time  and  satisfaction  in  results  that  come  from  observance  of  this 
precaution.  As  this  first  precipitate  invariably  contains  magnesia, 
as  well  as  some  lime  and  alkalies,  its  solution  and  reprecipitation, 
at  least  once,  are  necessary  in  all  cases. 

With  the  platinum  spatula  a  side  of  the  filter  is  loosened  and  a 
channel  made  between  the  filter  and  funnel,  so  that  all  the  liquid 
in  the  suction-tube  and  tubular  part  of  the  funnel  may  run  out 
into  the  beaker  below.  The  uncleaned  stirring-rod  remains  in 
the  400-c.c.  beaker  and  this  is  placed  conveniently  near  the  edge 
of  the  table.  The  funnel  is  removed  from  the  stand,  and  with  the 
platinum  spatula  the  filter  is  gently  loosened  all  around,  the  edge 
being  turned  down  as  little  as  possible,  and  the  paper  not  being 
torn.  The  funnel  is  then  held  with  its  side  horizontal  and  the 
folded  part  of  the  filter  underneath,  the  spatula  slipped  beneath 
this,  and  the  filter  with  its  contents  is  carefully  removed  from  the 
funnel,  and  placed  on  the  rear  inside  wall  of  the  400-c.c.  beaker, 
held  sloping  in  the  left  hand.  The  upper  edge  of  the  filter  should 
be  near  the  rim  of  the  beaker.  The  filter  is  now  unfolded,  opened 
out  and  gently  spread  and  pressed  against  the  glass,  which  should 
be  accomplished  without  tearing  the  paper. 

The  main  mass  of  the  precipitate  is  pushed  down  to  the  bottom 
of  the  beaker  with  the  spatula,  and  this  washed  off  with  a  few  jets 
of  water.  About  50  c.c.  of  water  is  added,  and  the  filter,  that  is 
now  adhering  to  the  rear  of  the  beaker,  is  washed  with  some  warm 
dilute  (1  :  1)  nitric  acid,  poured  in  several  small  portions  so  as  to 
dissolve  completely  the  adherent  precipitate,  but  not  disturb  the 


154  METHODS 

filter.  More  of  the  acid  is  added  to  that  in  the  bottom  of 
beaker,  about  25  c.c.  in  all  being  ample. 

At  this  point,  if  but  one  reprecipitation  is  to  be  made,  two  or 
three  cubic  centimeters  of  a  suspension  of  macerated  filter  paper 
(p.  51)  is  to  be  added.  If  another  precipitation  is  to  follow,  this 
addition  is  made  before  the  final  one.  The  addition  of  macerated 
paper  was  first  suggested  by  Dittrich,1  and  is  to  be  recommended. 
Some  experiments  by  Dr.  H.  S.  Roberts  and  myself  indicate  the 
efficiency  of  its  action.  The  object  of  this  addition  is  to  distribute 
fibers  of  cellulose  through  the  mass  of  precipitate,  so  that  on 
ignition  the  oxides  will  be  left,  not  as  a  very  hard  and  tough  lump, 
but  in  a  fine  and  porous  state  of  division,  thus  permitting  the  ready 
reoxidation  of  any  reduced  ferric  oxide,  and  also  greatly  facilitating 
the  subsequent  solution  in  pyrosulphate. 

The  liquid  is  now  heated  until  all  the  precipitate  is  dissolved, 
and,  when  it  is  almost  boiling,  slightly  diluted  ammonia  water 
is  added  in  very  small  excess,  methyl  orange  or  methyl  red  being 
used  as  an  indicator  as  before.  The  filter  is  to  be  washed  down 
with  the  ammonia.  The  ammonia  should  be  added  cautiously  as 
the  reaction  with  the  heated  and  rather  strong  acid  is  apt  to  -be  so 
violent  as  to  cause  loss  by  spattering. 

The  contents  of  the  beaker,  kept  well  stirred  (the  filter  still 
adhering  to  the  back  wall),  are  now  filtered  through  a  fresh  11-cm. 
filter,  placed  in  the  funnel  previously  used,  into  the  original  800-c.c. 
beaker. 

If  the  rock  contains  much  magnesia  or  lime,  as  with  the  diorites, 
gabbros,  basalts,  tephrites,  and  peridotites,  a  second  solution  in 
nitric  acid  and  reprecipitation  is  to  be  made,  this  being  carried 
out  exactly  as  before,  the  macerated  paper  being  now  added.  In 
this  case  the  washing  of  the  second  precipitate  need  not  be  thor- 
ough. It  may  occasionally  happen  that  a  third  reprecipitation  is 
called  for,  if  the  rock  is  exceptionally  high  in  magnesia,  or  in  lime 
if  the  persulphate  has  been  used,  but  this  will  seldom  be  necessary. 

After  the  final  precipitation,  whether  it  be  the  second  or 
third,  as  much  of  the  precipitate  as  is  easily  possible  is  to  be 
got  directly  on  the  filter.  The  sides  of  the  beaker  are  washed 
down  with  strong  jets  of  hot  water,  so  as  to  loosen  the  adhering 
precipitate,  but  without  disturbing  the  filter  or  filters  spread  out 
1  Dittrich,  pp.  10,  11,  14. 


ALUMINA  PRECIPITATE  155 

over  the  rear  wall  of  the  beaker.  The  washings,  with  the  precip- 
itate that  they  carry,  are  transferred  to  the  filter. 

In  order  to  remove  the  last  portions  of  precipitate,  which  cling 
tenaciously  to  the  walls  of  the  beaker,  the  filter  paper  hitherto 
spread  on  the  rear  wall  is  used  as  a  cleaner  or  "  policeman."  The 
lower  third  of  this  paper,  which  is  most  covered  with  precipitate, 
is  torn  off  with  the  end  of  the  stirring-rod,  and  is  rubbed  over  that 
side  of  the  beaker.  It  is  then,  along  with  the  loosened  precipitate, 
washed  into  the  filter.  Another  piece  is  then  torn  off  and  used  in 
the  same  way,  the  lower  part  of  the  stirring-rod  being  cleaned  by 
rubbing  it  against  the  mass  of  moist  paper  in  the  lip  of  the  beaker. 
Three  or  four  such  rubbings  will  serve  to  use  up  all  the  previous 
filters,  all  portions  of  which  must  be  got  with  all  the  precipitate 
into  the  filter  in  the  funnel,  and  render  the  interior  of  the  beaker 
perfectly  clean. 

The  contents  of  the  filter  are  then  washed  with  many  (10  or  12 
will  usually  suffice)  portions  of  hot  water,  during  which  the  mass  is 
gradually  brought  down  and  collected  in  the  lower  part  of  the 
filter  and  the  upper  zone  of  this  cleaned  of  precipitate.  This 
collection  of  the  precipitate  in  the  bottom  of  the  filter  and  the 
cleaning  of  one-quarter  or  one-half  of  a  centimeter  around  the 
edge  is  of  practical  importance,  as  it  greatly  facilitates  the  subse- 
quent transfer  to  a  crucible. 

For  ordinary  work,  in  a  third  precipitation  only  the  rinsings 
and  first  washings  need  be  caught  in  the  beaker,  as  the  amount  of 
magnesia  or  lime  in  the  final  washings  would  be  inappreciable,  and 
as  these  washings  would  add  considerably  to  the  bulk  of  liquid. 
The  contents  of  the  beaker  are  to  be  kept  (covered)  for  the  de- 
termination of  lime  and  magnesia  (p.  177). 

"  Basic  Acetate  "  Precipitation.1 — The  difficulties,  uncertain- 
ties, and  dangers  of  this  method  have  already  been  pointed  out,2 
and  once  more  is  it  strongly  urged  that  it  be  not  used,  except  in 
some  unusual  cases,  where  the  ordinary  method  of  precipitation 
by  ammonia  will  not  serve.  It  is  described  here,  somewhat 
reluctantly,  partly  because  it  may  rarely  happen  to  be  the  best 
method  available,  as  when  there  is  much  manganese  present,  and 

1  Classen,  1,  p.  465;   Fresenius,  1,  p.  647;  Hillebrand,  p.  100;   Mellor,  p. 
362;  Treadwell,  2,  p.  152. 

2  Cf .  pp.  149-150. 


156  METHODS 

because,  being  a  method  so  often  recommended,  the  description 
may  aid  the  student  in  avoiding  some  of  its  pitfalls. 

To  the  cold  filtrate  from  the  silica,  which  contains  a  little  free 
acid,  and  whose  volume  is  about  200  c.c.,  a  concentrated  solution 
of  sodium  carbonate  is  added  cautiously  till  the  fluid  turns  a  dark 
red  and  a  slight  turbidity  is  observed,  which  does  not  disappear  on 
stirring.  This  addition  may  be  made  in  the  beaker  covered  with 
a  watch-glass,  and  the  solution  of  carbonate  is  introduced  through 
the  small  funnel  with  bent  tip,  so  as  to  avoid  loss  by  effervescence. 
The  watch-glass,  tip  of  the  funnel,  and  the  sides  of  the  beaker  are 
rinsed  down,  and  if  these  rinsings  are  sufficiently  acid  to  redissolve 
the  slight  precipitate,  as  may  sometimes  happen,  a  few  more 
drops  of  carbonate  solution  are  added  till  a  slight  permanent  pre- 
cipitate is  formed  again. 

Dilute  hydrochloric  acid  is  then  added,  drop  by  drop  and  very 
cautiously,  with  constant  stirring,  till  the  slight  precipitate  and 
turbidity  just  disappear,  but  the  fluid  still  retains  its  deep-red  color. 
Especial  caution  is  needed  here,  as  any  decided  excess  will  set  free 
enough  extra  acetic  acid  from  the  sodium  acetate  added  subse- 
quently to  render  the  precipitation  of  alumina  and  iron  incomplete. 
If  too  much  has  been  added,  therefore,  the  solution  is  once  more 
to  be  slightly  more  than  neutralized  with  sodium  carbonate  and 
again  treated  with  dilute  hydrochloric  acid  more  cautiously. 

Enough  acetic  acid  of  specific  gravity  1.044  (33  per  cent)  is 
poured  in  to  form  about  3  per  cent  by  volume  of  the  total  liquid, 
preferably  rather  less  than  more.  As  the  final  volume  will  be 
about  300  c.c.,  8  or  at  most  10  c.c.  of  acetic  acid  are  sufficient.  If 
'too  little  is  present  a  slight  precipitation  of  manganese  is  to  be 
feared,  while  if  too  much  free  acid  is  present  alumina  and  iron  will 
not  be  completely  thrown  down,  but  will  pass  in  small  amount 
into  the  filtrate. 

About  2  grams  of  sodium  acetate  dissolved  in  a  little  water 
are  then  added.  This  is  the  amount  for  most  rocks,  but  it  may 
be  varied  somewhat  with  advantage.  Thus  for  rocks  low  in 
the  sesquioxides,  as  granites  and  rhyolites,  1J  grams  may  serve, 
though  2  will  not  be  amiss.  But  in  such  rocks  as  foyaites, 
phonolites,  gabbros,  basalts,  or  tephrites,  which  contain  large 
amounts  of  these  oxides,  the  quantity  would  best  be  increased 
to  3  grams,  which  may  be  considered  the  limit. 


ALUMINA   PRECIPITATE  157 

If  the  liquid  has  not  a  volume  of  300  c.c.,  it  is  diluted  to  this 
bulk,  or  to  350  c.c.  if  the  larger  amount  of  sodium  acetate  has  been 
used.  It  is  heated  to  boiling  and  allowed  to  boil  for  not  more  than 
a  minute  or  two,  as  prolonged  boiling  renders  the  precipitate  slimy 
and  difficult  to  filter.  After  settling  for  a  few  minutes,  the  liquid 
is  filtered  through  an  11-cm.  filter,  and  washed  only  two  or  three 
times  with  hot  water.  This  precipitate,  which  consists  of  basic 
acetates  of  aluminum  and  iron,  with  the  titanium,  zirconium, 
chromium  and  phosphorus  of  the  rock,  is  rather  more  apt  to  run 
through  the  filter  than  the  precipitate  of  hydroxides  produced  by 
ammonia.  The  washing,  therefore,  should  not  be  thorough,  and 
it  is  as  well  to  add  a  little  sodium  acetate  to  the  hot  washing-water, 
so  as  to  have  a  crystalline  salt  present. 

After  this  slight  washing  the  precipitate  is  dissolved  in  hydro- 
chloric acid  by  the  method  just  described,  reprecipitated  with 
ammonia  water,  and  this  solution  and  reprecipitation  repeated 
if  the  rock  demands  it,  exactly  as  was  done  in  the  method  by 
ammonia  alone.  The  final  precipitate  is  to  be  ignited  as  described 
below.  It  must  be  remembered,  however,  that  there  will  probably 
be  another  filter  containing  the  alumina  and  iron  which  have 
passed  through  with  the  filtrate,  so  that  the  ignition  of  the  main 
portion  must  wait  till  this  has  been  incinerated  with  the  extra 
filters,  to  avoid  reduction  of  the  ferric  oxide.  Otherwise  ignition 
in  a  separate  crucible  and  consequently  two  fusions  with  potas- 
sium pyrosulphate  are  involved.  The  filtrate  is  reserved  for  the 
determination  of  lime  and  magnesia  (p.  177). 

Ignition  of  the  Precipitate. — The  moist  final  precipitate  in  the 
filter  is  allowed  to  drain  well  and,  if  time  permits,  is  advanta- 
geously kept  for  half  an  hour  or  so,  the  funnel  covered  with  a 
filter  paper,  so  as  to  dry  out  somewhat.  It  is  then  placed  moist 
in  the  crucible  which  has  been  used  for  the  determination  of  silica, 
and  in  which  there  still  remain  the  impurities  left  on  evaporation 
with  hydrofluoric  acid.  This  is  done  with  the  aid  of  the  platinum 
spatula,  the  free  edges  of  the  filter  being  folded  over  inwards,  the 
filter  very  gently  freed  from  the  funnel  without  tearing  the  paper, 
and  the  whole  carefully  placed  in  the  crucible  with  the  three-fold 
portion  uppermost.  Care  should  be  taken  not  to  soil  the  sides  of 
the  crucible,  and  to  leave  abundant  free  passage  for  the  exit  of 
steam  from  beneath  the  mass.  The  spatula  and  the  interior  of 


158  METHODS 

the  funnel  are  to  be  cleaned  with  a  small  piece  of  filter  paper, 
which  is  laid  on  top  of  the  package  in  the  crucible. 

The  drying  of  the  moist  mass  must  be  done  very  cautiously, 
at  a  considerable  height  (8  inches  or  so)  above  a  small  flame, 
the  crucible  being  vertical  and  covered.  Constant  watching 
is  necessary  at  first  to  prevent  any  bubbling  of  the  pasty  mass, 
which  would  soil  the  upper  portion  of  the  crucible  with  precipitate 
and  render  difficult  its  complete  solution  in  fused  pyrosulphate. 
The  crucible  is  very  gently  and  cautiously  lowered  as  the  mass 
dries  off,  until  the  filter  is  carbonized,  when  it  is  heated  vertically 
for  a  short  time  at  a  bright-red  heat  until  the  cover  is  free  from 
adhering  carbon. 

The  crucible  is  then  laid  on  its  side  on  the  platinum  triangle, 
the  mouth  at  the  twisted  end,  and  the  cover  is  leant  against  it, 
almost  vertical,  with  its  upper  edge  a  little  below  the  top  of  the 
crucible,  leaving  a  narrow  opening  above  and  below. 

The  flame  is  directed  against  the  bottom  and  lower  third  of 
the  crucible,  the  flame  not  being  violent  enough  to  cause  dangerous 
draughts,  and  the  incineration  of  the  paper  is  quickly  accomplished. 
It  is  then  heated  at  a  bright-red  heat  for  at  least  twenty  minutes. 
This  will  insure  complete  incineration  of  the  filter  and  the  reoxida- 
tion  of  any  ferrous  oxide  that  may  have  been  formed,  which  will  be 
much  easier  and  more  complete  if  macerated  paper  has  been  added 
before  the  final  precipitation.  As  the  last  portions  of  water 
are  not  always  completely  expelled  by  the  heat  of  a  Bunsen  or 
Meker  burner,  it  is  best  to  blast  for  ten  minutes,1  the  crucible 
being  vertical  and  covered,  and  the  flame  playing  only  on  the 
bottom  of  the  crucible. 

After  cooling  in  the  desiccator,  the  crucible  is  weighed,2  and 
the  difference  between  this  and  the  weight  of  the  empty  crucible, 
obtained  prior  to  the  ignition  of  the  silica  (p.  143),  is  that  of  the 
AbOa,  total  iron  as  Fe2Oa,  MnsO^  TiO2,  Zr(>2,  P2Os  and  a  trace 
of  SiO2.  This  may  be  noted  as  Al2O3+Fe203+£.  The  amounts 
of  these  various  constituents  are  determined  separately,  and  that 

1  Blum  (Jour.  Am.  Chem.  Soc.,  38,  p.  1293,  1916)  has  shown  that  blasting 
for  ten  minutes  is  as  effective  as  for  half  an  hour. 

2  Blum  (op.  cit.,  p.  1292)  has  shown  that  even  the  blasted  alumina  is 
markedly  hygroscopic,  so  that  it  is  necessary  to  keep  the  crucible  closely  cov- 
ered while  in  the  desiccator  and  during  the  weighing. 


ALUMINA  PRECIPITATE  159 

of  alumina  by  difference.  If  three  11-cm.  filters  have  been  used, 
it  is  best  to  subtract  their  weight  also  from  that  of  the  ignited 
precipitate. 

The  ignited  precipitate  in  the  crucible  is  used  for  jbhe  deter- 
mination of  total  iron,  titanium  dioxide  and  the  trace  of  silica, 
its  solution  being  effected  by  fusion  with  potassium  pyrosulphate. 
This  process  may  be  advantageously  begun  immediately  after 
weighing,  as  it  takes  several  hours. 

Fusion  with  Pyrosulphate.1 — The  ignited  ammonia  precipitate 
is  very  complex.  It  contains,  with  all  rocks,  all  of  the  alumina, 
ferric  oxide,  titanium  dioxide,  phosphorus  pentoxide,  and  man- 
ganous  oxide  if  persulphate  was  added  before  the  precipitation,  as 
well  as  the  small  amount  of  silica  that  has  escaped  the  first  evapo- 
rations to  dryness;  and  with  some  rocks  it  contains  also  all  the 
chromium,  vanadium,  zirconia  and  the  rare  earths.  If  two  or 
three  solutions  and  reprecipitations  of  the  ammonia  precipitate 
have  been  made,  it  also  is  contaminated  with  the  small  amount  of 
ash  derived  from  the  filter  papers  which  may  not  be  negligible 
in  accurate  analyses.  Of  these  constituents  only  the  ferric  oxide 
(which  includes  the  oxidized  ferrous  oxide  of  the  rock),  the  titanium 
dioxide,  and  the  silica  are  determined  in  this  precipitate,  the 
others  being  determined  in  separate  portions  and  duly  allowed  for 
in  order  to  arrive  at  the  weight  of  the  alumina. 

The  precipitate  is  best  brought  into  solution  by  fusion  with 
potassium  pyrosulphate,  which  operation  is  carried  out  as  follows: 

Four  or  five  grams  of  lumps  or  coarsely  powdered  potassium 
pyrosulphate  are  placed  in  the  crucible  containing  the  precipitate, 
so  as  to  completely  cover  this,  and  care  being  taken  to  avoid 
mechanical  loss  of  the  precipitate.  The  amount  used  will  vary 
somewhat  with  each  rock,  but  those  mentioned  will  usually  be 
sufficient.  In  general,  it  is  hardly  necessary  to  weigh  out  the 
pyrosulphate,  but  to  add  enough  to  fill  the  crucible  about  one- 
third. 

The  crucible  is  placed  over  a  low  flame,  and  heated  gently  till 
the  salt  is  fused.  It  is  then  raised  to  a  distance  above  the  flame 
(about  20-30  cm.),  where  the  pyrosulphate  will  remain  in  a  state 
of  fusion  and  the  moisture  which  it  contains  will  be  driven  off, 
without  any  boiling  or  spattering  against  the  crucible  cover.  With 

1  Mellor,  p.  184.     Hillebrand  (p.  105),  adopts  a  somewhat  different  method. 


160  METHODS 

some  practice  the  height  can  be  adjusted  easily,  and  this  point 
is  an  important  one  to  attend  to,  as  any  drops  on  the  cover  or  the 
upper  sides  tend  to  spread  on  further  heating  and  run  over  the 
edges,  leading  to  loss  of  iron.  The  whole  process  at  this  stage 
must  be  carefully  watched  to  guard  against  this  mishap. 

It  may  happen,  even  if  great  care  is  taken,  that  there  is  a  slight 
creep  of  the  salt  around  the  rim  and  about  the  cover.  This  is 
almost  always  caused  by  too  intense  or  too  rapid  heating.  This 
deposit  should  not  be  touched  when  moving  the  crucible  to  the 
slab  or  disturbed  during  the  cooling. 

In  the  course  of  half  an  hour  or  so  all  water  will  be  driven 
off,  and  the  crucible  can  be  lowered  gradually  till  it  is  immediately 
above  the  small  flame,  where  it  is  kept  for  another  hour  or  so. 
Here  also  the  contents  should  be  watched  at  intervals  to  see  that 
there  is  no  spattering.  The  precipitate  has  been  gradually  dis- 
solving and  the  fused  salt  become  darker  in  color.  The  larger 
lumps  of  oxide  stay  at  the  bottom,  while  some  may  float  on  the  top 
of  the  liquid. 

When  the  greater  part  of  the  floating  portion  has  dissolved, 
any  small  particles  which  may  be  adhering  to  the  sides  above  the 
level  of  the  liquid  may  be  washed  down  by  a  slight  rotary  motion 
of  the  crucible.  The  flame  is  then  raised  a  little.  This  more 
intense  heating  should  be  carried  out  with  caution  to  avoid  boiling, 
and,  until  the  last  stages,  the  bottom  of  the  crucible  should  not 
be  allowed  to  become  red-hot.  White  vapors  of  sulphur  trioxide 
are  given  off,  and  the  crucible  is  examined  every  now  and  then 
till  all  the  floating  precipitate  has  been  dissolved.  If  any  par- 
ticles obstinately  adhere  above  the  liquid,  the  crucible  may  be 
held  obliquely  in  the  triangle,  so  as  to  let  the  fused  salt  act  on  these. 

The  heat  is  then  increased  somewhat  until  the  bottom  of  the 
crucible  is  a  faint  red,  the  liquid  getting  thicker  through  loss  of 
sulphur  trioxide  and  the  formation  of  the  more  difficultly  fusible 
normal  potassium  sulphate.  The  liquid  mass  becomes  also  a 
very  dark  brown,  almost  opaque  if  much  iron  is  present,  the 
depth  of  color  increasing  with  the  temperature.  This  is  due  to  the 
greater  dissociation  with  increasing  temperature,  and  the  conse- 
quent larger  proportion  of  yellow  or  brown  iron  ions. 

The  bottom  of  the  crucible  may  be  examined,  notwithstanding 
the  opacity  of  the  liquid,  to  see  if  all  the  precipitate  has  been  dis- 


ALUMINA  PRECIPITATE  161 

solved,  by  removing  the  flame,  and  allowing  the  crucible  to  cool 
with  the  cover  off.  The  fused  mass  will  gradually  become  less 
opaque  and  lighter  in  color,  till  it  is  transparent  enough  to  be  seen 
through  before  solidification  commences  at  the  surface.1 

When  no  more  undissolved  substance  is  visible,  the  heating 
at  a  low  red  heat  is  continued  for  half  an  hour,  to  render  complete 
solution  certain,  and  the  crucible  is  placed  on  a  stone  or  iron  slab 
to  cool.  This  cake  loosens  from  the  crucible  far  more  readily  than 
that  of  the  sodium  carbonate  fusion,  often  with  a  ringing  sound, 
and  it  also  usually  cracks,  so  that  it  offers  no  difficulty  in  removal. 

It  may  seem  that  this  process  calls  for  almost  constant  atten- 
tion and  that  it  takes  an  inordinate  amount  of  time.  In  reality, 
however,  after  one  has  had  a  little  practice  in  adjusting  the  heat 
at  the  various  stages,  only  an  occasional  glance  is  necessary,  and 
the  whole  can  often  be  accomplished  in  from  three  to  four  hours, 
especially  if  macerated  paper  has  been  used,  so  that  the  oxides 
are  in  the  form  of  a  powder.  This,  moreover,  is  of  no  great 
importance,  as  the  analyst  can  be  busy  with  other  parts  of  the 
analysis. 

When  the  mass  is  thoroughly  cold,  any  deposit,  due  to  creeping, 
both  on  the  crucible  rim  and  on  the  cover,  is  washed  off  with  a 
very  little  hot  water  into  a  250-c.c.  beaker.  Water  is  then  poured 
into  the  crucible  from  the  wash-bottle,  enough  to  about  half  fill 
the  crucible,  and  it  is  gently  heated  till  the  cake  loosens,  when 
this  is  transferred  by  means  of  the  platinum  spatula  to  the  250- 
c.c.  beaker.  The  crucible  is  well  washed  with  hot  water  into 
the  same  beaker,  until  all  adhering  sulphate  is  removed,  and  the 
cover  is  treated  likewise.  The  final  volume  of  liquid  in  the  beaker 
may  be  about  100  c.c.  About  10  c.c.  of  concentrated  sulphuric 
acid  are  added,  not  only  to  facilitate  the  solution,  but  to  prevent 
reversion  to  or  precipitation  of  metatitanic  acid,  which  would 
diminish  the  apparent  amount  of  titanium  dioxide  determined 
later  by  the  colorimetric  method.  The  beaker  is  then  heated  over 
a  low  flame  or  on  the  water-bath  till  solution  is  complete,  except 
for  traces  of  silica,  which  is  practically  insoluble  in  the  melted 
potassium  pyrosulphate. 

The  contents  of  the  beaker  are  filtered  through  a  7-cm.  filter 
into  a  250-c.c.  Erlenmeyer  flask,  the  first  beaker  being  well  rinsed 

1  This  is  more  easy  with  potassium,  than  with  sodium  pyrosulphate. 


162  METHODS 

at  least  half  a  dozen  times.  The  filter  is  also  well  washed.  If  the 
fusion  has  been  successful,  a  few  flakes  of  silica  only  will  be  found 
in  the  filter.  If  not  it  will  also  contain  small,  dark  particles  of 
undissolved  oxide. 

In  any  case,  it  is  placed  in  an  unweighed,  small  crucible,  car- 
bonized at  a  gentle  heat,  strongly  ignited  and  weighed.  A  drop  of 
dilute  sulphuric  acid  and  two  or  three  of  hydrofluoric  acid  are 
added,  driven  off  by  gentle  heating,  the  crucible  again  ignited 
and  weighed.  The  loss  in  weight  represents  the  trace  of  silica, 
which  is  to  be  added  to  that  of  the  main  portion,  already  deter- 
mined (p.  146).  It  will  seldom  amount  to  more  than  a  milligram. 

The  residue  left  in  the  crucible,  which  will  contain  a  little 
iron  or  titanium  oxides,  is  dissolved  by  fusion  with  a  small  lump 
of  acid  potassium  sulphate,  which  is  quickly  effected.  After 
cooling,  this  is  dissolved  in  the  crucible  in  a  little  warm  water 
containing  a  drop  of  sulphuric  acid,  and  the  solution  poured  into 
that  in  the  250-c.c.  flask  the  crucible  being  also  rinsed  into  this. 

8.  TOTAL  IRON  OXIDES  1 

The  solution  of  the  pyrosulphate  fusion,  obtained  as  above,  is 
used  for  the  determination  of  the  total  iron  oxides  as  ferric  oxide 
and  for  that  of  titanium  dioxide.  For  the  former  the  ferric  sul- 
phate is  reduced  to  ferrous  salt  and  titrated  with  permanganate; 
for  the  latter  a  colorimetric  method  is  almost  invariably  used. 

Iron  and  titanium  are  precipitated  quantitatively  and  com- 
pletely separated  from  aluminum  and  manganese  by  the  new 
reagent  "  cupferron";2  and  titanium  may  be  separated  from  iron 
by  a  method  suggested  by  Thornton.3  At  the  present  time  it  must 
suffice  to  refer  to  these  methods  as  available  in  certain  cases. 

Errors. — The  most  frequent  and  most  serious  error  involved 
in  the  iron  determination  is  incomplete  reduction  of  the  ferric 
oxide.  This  can  be  guarded  against  by  allowing  the  gas,  whether 
hydrogen  sulphide  or  sulphur  dioxide,  to  pass  for  a  longer  time  than 

1  Classen,  2,  pp.  454-455;  Gooch,  pp.  141-146;  Hillebrand,  pp.  107-109; 
Mellor,  pp.  451-453;  Treadwell,  2,  pp.  607-610. 

2Cf.  Treadwell,  2,  pp.  838-842. 

3W.  M.  Thornton,  Jr.,  Am.  Jour.  Sci.,  37,  p.  173,  1914,  and  ditto,  37,  p. 
407,  1914. 


TOTAL  IRON  OXIDES  163 

that  which  experience  has  shown  to  be  generally  necessary  and  by 
not  boiling  down  to  a  small  bulk.  It  can  also  be  prevented  by 
testing  the  solution  for  the  presence  of  ferric  salt  when  reduction 
is  assumed  to  be  complete. 

i  A  second  source  of  error  is  deterioration  in  the  strength  of  the 
permanganate  solution,  which  can  be  avoided  by  keeping  it  under 
proper  conditions  and  frequently  standardizing  it  (p.  52).  These 
precautions  are  often  not  sufficiently  observed. 

Zinc  is  not  recommended  as  a  reducing  agent,1  partly  because 
perfectly  pure  and  iron-free  zinc  is  very  difficult  to  procure,  partly 
because  of  the  difficulty  in  ascertaining  when  reduction  is  com- 
plete, partly  because  I  do  not  consider  that  it  offers  any  note- 
worthy advantage  over  the  method  adopted  here,  but  above  all 
because  of  the  reducing  effect  of  nascent  hydrogen  on  titanic 
sulphate,  which  is  always  present,  and  on  platinic  and  vanadic 
salts.  The  lower-oxides  of  all  three  would  affect  the  permanganate 
and  thus  appear  as  ferric  oxide.  With  many  rocks  the  amount  of 
titanium  present  is  so  high  that  this  would  be  a  serious  error. 

With  proper  precautions,  errors  involved  during  the  titration, 
such  as  oxidation  by  atmospheric  oxygen,  are  negligible. 

Reduction  of  Ferric  to  Ferrous  Oxide. — The  filtered  solution 
of  the  mixed  sulphates  contains  all  the  iron  in  the  ferric  state. 
This  has  to  be  reduced  to  ferrous  for  titration  with  potassium 
permanganate,  to  determine  the  total  iron  oxide. 

As  has  been  noted,  the  use  of  zinc  for  this  purpose  is  not  to  be 
recommended,  and  the  best  reagent  is  hydrogen  sulphide.  This 
commends  itself  on  account  of  its  certainty  and  rapidity  of  action, 
its  easy  and  complete  removability,  and  still  more  by  the  fact 
that  it  has  no  reducing  action  on  the  titanic  and  platinic  sulphates 
that  are  always  present.  Sulphur  dioxide  is  often  recommended  2 
and  employed,  but  is  not  so  suitable  for  rock  analysis  because  of  its 
action  on  the  platinum  taken  up  from  the  crucible  and  basin. 
This  is  reduced  to  the  platinous  condition  by  sulphur  dioxide,  while 

1  Mellor  (p.  187)  advocates  its  use,  an  opinion  in  which  I  differ  with  him. 
If  it  be  used,  Gooch  and  Newton  (Am.  Jour.  Sci.,  23,  p.  365,  1907)  recommend 
the  use  of  cupric  sulphate  to  oxidize  the  titanium  after  reduction  by  zinc. 
This,  however,  does  not  remove  other  objections  to  the  zinc  reduction  method. 

2  Mellor  (p.  191)  recommends  ammonium  bisulphite,  but  this  salt  is  not 
very  stable  on  keeping  and  offers  no  special  advantage  over  the  gas. 


164  METHODS 

it  is  precipitated  by  hydrogen  sulphide  and  can  thus  be  eliminated. 
The  error  so  involved  in  the  use  of  sulphur  dioxide  will,  of  course, 
not  be  great,  but  may  as  well  be  avoided.  Hydrogen  sulphide  is 
much  more  readily  obtainable  than  sulphur  dioxide — a  minor,  but 
practically  important,  consideration. 

A  current  of  hydrogen  sulphide,  washed  with  water,  in  the 
usual  manner,  is  allowed  to  bubble  at  the  rate  of  a  bubble  a 
second,  through  the  solution  in  the  250-c.c.  beaker,  which  is 
covered  with  a  4-inch  watch-glass,  the  gas  being  introduced 
through  a  bent  glass  tube,  passing  through  the  lip  and  reaching 
to  the  bottom.  Although  Hillebrand  recommends  that  the 
solution  be  hot,  I  have  not  found  this  necessary,  and  pass  the  gas 
through  the  cold  solution.  The  current  is  continued  till  reduction 
is  complete,  which  is  indicated  by  the  turbid  liquid  becoming 
clear  and  masses  of  sulphur  coagulating,  which  are  stained  brown 
by  traces  of  platinum  sulphide,  derived  from  the  basin  and 
crucibles.  At  least  twenty  minutes  should  be  allowed  for  this, 
as,  if  the  reduction  is  incomplete,  the  amount  of  total  iron  will 
be  too  low,  and  that  of  alumina  too  high.1 

The  glass  tube  through  which  the  gas  has  been  introduced 
is  rinsed  off  into  the  flask,  and  the  contents  are  filtered  off  through 
a  7-cm.  filter  into  a  400-c.c.  Erlenmeyer  flask.  This  is  to  be  done 
as  quickly  as  possible,  and  the  filter  kept  full.  The  washing  is 
carried  out  with  water  containing  some  H^S,2  six  or  eight  rinsings 
of  the  smaller  flask  and  passage  through  the  filter  being  sufficient. 
Owing  to  the  presence  of  finely  divided  sulphur,  the  filtrate  always 
becomes  opalescent.  But  this  need  cause  no  concern,  as  the  sul- 
phur is  completely  oxidized  in  the  subsequent  boiling,  by  the  sul- 
phuric acid  present,  whereupon  the  liquid  becomes  perfectly  clear. 

The  solution  of  the  small  cake  of  fused  sulphate  containing 
the  residue  from  the  trace  of  silica  is  poured  in  if  it  has  not  been 
added  before,  and  the  crucible  is  washed  once  or  twice,  the  excess 
of  H2S  present  being  more  than  sufficient  for  the  complete  reduc- 
tion of  the  ferric  sulphate  which  it  contains. 

If  sulphur  dioxide  be  used,  the  gas  should  be  washed  through 

1  The  time  needed  will,  of  course,  vary  with  the  amount  of  iron  oxide  in  the 
rock.     One  half-hour  is  usually  ample. 

2  This  is  conveniently  made  in  a  150-c.c.  beaker  after  the  reduction  is  fin- 
ished. 


TOTAL  IRON  OXIDES  165 

a  small  volume  of  water,  and  the  acid  solution  should  be  pre- 
viously nearly  neutralized,  by  dropping  in  slowly  a  concentrated 
solution  of  sodium  carbonate.  This  is  another  disadvantage  in 
the  use  of  this  gas,  as  it  complicates  the  determination  of  titanium 
by  adding  to  the  amount  of  alkali  sulphate  present. 

It  will  now  be  best  to  test  the  liquid  to  ascertain  if  reduction  is 
complete.  This  may  be  done  by  taking  out  a  drop  at  the  end  of  a 
stirring-rod  and  adding  it  to  a  few  drops  of  potassium  thiocyanate 
solution  in  a  watch-glass  resting  on  a  white  porcelain  plate.  If  a 
red  color  appears,  hydrogen  sulphide  should  be  passed  through 
the  liquid  in  the  400-c.c.  flask  for  another  fifteen  minutes.  It  will 
not  be  necessary  to  filter  after  this. 

Four  or  five  small  pieces  of  platinum  foil,  bent  at  right  angles, 
or  small  pieces  of  a  broken  porcelain  crucible,  are  then  dropped  in, 
so  as  to  prevent  bumping  on  boiling. 

The  flask  is  then  placed  over  a  flame,  and  a  carbon-dioxide 
generator  set  in  action,  the  gas  being  freed  from  possible  H^S 
(due  to  sulphides  in  the  marble)  by  passing  through  a  column  of 
pumice  soaked  in  copper  sulphate  solution,  and  washed  by  a  wash- 
bottle  containing  water.  The  carbon  dioxide  is  allowed  to  bubble 
at  the  rate  of  several  bubbles  a  second,  and  is  passed  through 
the  liquid  by  a  bent  piece  of  glass  tubing.  Complete  saturation  by 
H2S  is  shown  by  small  bubbles  of  this  gas  rising  in  the  liquid 
soon  after  the  heating  begins,  and  long  before  it  has  become  hot 
enough  to  simmer  or  boil. 

The  flow  of  carbon  dioxide  is  still  kept  up  after  boiling  has 
begun,  and  the  boiling  continued  briskly  until  the  expulsion  of 
hydrogen  sulphide  is  complete,  as  shown  by  occasional  testing  of 
the  issuing  steam  with  filter  paper  dipped  in  lead  acetate  solution. 
This  process  will  take  about  twenty  minutes,  unless  much  opales- 
cent sulphur  is  present,  when  possibly  half  an  hour  will  be  needed 
to  clear  the  liquid.  There  is  no  danger  of  reoxidation  of  the  fer- 
rous sulphate,  since  at  first  the  liquid  contains  hydrogen  sulphide, 
and  later  the  boiling  is  carried  on  in  an  atmosphere  of  steam  and 
carbon  dioxide. 

The  boiling  off  of  the  reducing  gas  should  not  be  carried  so  far 
as  to  diminish  the  liquid  to  less  than  one-half  of  its  original 
volume,  as  hot,  strong  sulphuric  acid  solution  has  an  oxidizing 
effect  on  ferrous  salts. 


166  METHODS 

The  flame  is  now  extinguished,  and  the  flask  grasped  by  the 
neck  with  a  towel.  While  the  stream  of  carbon  dioxide  is  still 
passing  into  the  flask,  it  is  placed  in  a  capacious  vessel,  such  as  a 
basin  or  large  casserole,  that  is  set  near  at  hand.  The  contents 
are  cooled  in  this,  either  with  a  stream  of  running  water,  or  more 
conveniently  and  quickly  by  surrounding  the  lower  part  of  the 
flask  with  cracked  ice.  The  stream  of  carbon  dioxide  is  continued 
during  the  cooling. 

If  sulphur  dioxide  has  been  used  for  reduction,  10  c.c.  of  dilute 
(1:1)  sulphuric  acid  are  to  be  added.  This  will  not  be  necessary 
if  hydrogen  sulphide  has  been  used. 

Titration  of  Iron.1 — While  the  liquid  in  the  Erlenmeyer  flask 
is  cooling  in  a  stream  of  carbon  dioxide,  the  preparations  may  be 
made  for  the  titration.  The  stock  bottle  of  permanganate  is 
shaken,  so  as  to  wash  down  any  drops  of  water  that  have  distilled 
onto  the  upper  part  of  the  bottle,  and  a  weight  burette  is  nearly 
filled  with  the  solution,  which  is  to  be  transferred  with  a  50-c.c. 
pipette,  and  not  poured  out,  as  this  would  possibly  disturb  the 
titer.  If  the  rock  is  known  to  contain  but  little  iron,  as  with  a 
granite,  a  trachyte,  or  a  rhyolite,  a  burette  of  50-c.c.  capacity 
will  answer;  otherwise  one  of  100-c.c.  is  to  be  taken,  and  this  is 
always  safer.  The  burette  is  weighed,  but  only  to  the  nearesl 
centigram,  as  this  corresponds  to  one-hundredth  of  a  cubic  centi- 
meter, less  than  one  drop,  and  so  is  exact  enough  (cf.  p.  35). 
The  burette  is  then  placed  in  a  clamp,  secured  firmly,  and  is  raised 
to  a  height  that  will  just  permit  slipping  the  Erlenmeyer  flask 
beneath  its  tip.  The  cap  that  guards  the  tip  is  removed,  and  the 
stopper  turned  so  that  the  small  perforations  in  it  and  in  the 
neck  coincide. 

If  an  ordinary  Mohr's  burette  is  used,  it  is  made  ready  as 
usual,  the  meniscus  being  brought  to  the  zero  line. 

When  the  contents  of  the  flask  are  quite  cold  it  is  placed  on  a 
white  porcelain  plate,2  its  mouth,  just  below  the  tip  of  the  burette, 
and  the  permanganate  is  dropped  slowly  into  the  liquid,  not  run- 

1  For  a  description  of  the  operation  of  titration,  see  page  105. 

2  In  moving  the  flask  the  hand  may  be  placed  over  its  mouth  so  as  to  keep 
the  carbon  dioxide  atmosphere  in  it.     The  gas  inlet  tube  may  be  left  in  or,  if 
removed,  is  slightly  washed  off  both  inside  and  outside,  the  washings  falling 
into  the  flask. 


TITANIUM   DIOXIDE  167 

ning  down  the  sides  of  the  flask.  The  flask  is  given  a  rotary  motion 
with  the  left  hand,  so  as  to  distribute  the  permanganate  and  lessen 
the  chance  of  splashing.  If  any  drops  fall  or  are  splashed  on  the 
sides  or  neck  they  are  washed  down  into  the  flask  with  cold,  boiled 
water.  With  a  little  practice  the  proper  manipulation  is  easily 
learned  and  the  titration  quickly  carried  out. 

When  the  amount  of  standard  solution  needed  is  roughly 
known,  about  half  of  this  may  be  added  quickly  in  portions  of 

1  or  2  c.c.  at  a  time,  with  rotation  to  disappearance  of  the  color 
after  each  addition.     Beyond  this,  the  permanganate  should  be 
added  by  drops,  with  constant  rotation  to  avoid  overruning  the 
mark.     When  the  color  begins  to  disappear  slowly,  single  drops 
are  to  be  added  with  great  caution,  till  one  of  them  produces  a 
pink  blush  throughout  the  liquid  which  does  not  vanish  on  stirring 
or  rotating  for  a  short  time.     As  very  dilute  solutions  of  perman- 
ganate are  unstable,  this  color  will  vanish  on  long  standing,  even 
when  the  reaction  is  complete.     After  waiting  a  few  moments 
after  the  addition  of  the  last  drop,  the  burette  is  wiped  off  and 
again  weighed. 

The  number  of  grams  of  permanganate  solution  used  is  then 
multiplied  by  the  amount  of  Fe2Os  equivalent  to  1  gram  of  the 
standard,1  the  product  giving  the  total  iron  in  the  rock  determined 
as  Fe20s.  From  this  is  to  be  deducted  later  the  iron  present  as 
FeO,  and  that  which  may  exist  as  Fe$2. 

After  titration  of  the  iron,  if  the  amount  of  TiO2  is  less  than 

2  per  cent,  the  solution  is  to  be  evaporated  on  the  water-bath 
down  to  about  150  c.c.  either  in  porcelain  or  platinum,  the  flask 
being  rinsed  well  and  the  rinsings  added  during  the  evaporation. 
This  liquid  is  to  be  placed  in  a  250-c.c.  measuring  flask,  with  glass 
stopper,  but  not  filled  to  the  mark,  and  reserved  for  the  determina- 
tion of  titanium  dioxide. 

9.  TITANIUM  DIOXIDE 

For  the  determination  of  titanium  dioxide  the  whole  volume  of 
solution  in  which  the  total  iron  has  been  titrated  is  best  adapted. 
This  contains  all  of  the  titanium  in  solution  as  sulphate,  and  with 

1  If  an  ordinary  burette  has  been  used,  cubic  centimeters  are  to  be  under- 
stood here  instead  of  grams. 


168  METHODS 

no  possible  traces  of  hydrofluoric  acid,  which  exerts  such  a  dele- 
terious effect  on  the  colorimetric  method.  If,  for  any  reason,  this 
solution  is  not  available,  the  titanium  dioxide  can  be  determined 
in  separate  portion  of  rock  powder,  one  gram  of  which  is  brought 
into  solution  by  evaporation  (under  the  hood),  in  a  platinum 
crucible  with  a  mixture  of  dilute  sulphuric  acid  (1:1)  and  hydro- 
fluoric acid.  This  is  continued  till  fumes  of  sulphuric  acid  are 
given  off,  but  not  to  dryness,  when  more  of  the  dilute  sulphuric 
acid  is  added,  and  the  evaporation  is  continued  till  there  are  no 
traces  of  hydrofluoric  acid,  which  may  take  four  or  five  repetitions 
and  additions  of  sulphuric  acid.  Or  the  solution  in  which  ferrous 
oxide  has  been  determined  will  answer,  if  it  is  evaporated  down 
(in  platinum)  repeatedly  with  sulphuric  acid,  to  expel  hydrofluoric 
acid  completely. 

There  are  two  very  different  methods — colorimetric  and  grav- 
imetric— by  which  titanium  dioxide  is  determined  in  rocks.  The 
former  is,  by  far,  the  most  accurate  and  most  expeditious,  is 
best  adapted  to  the  small  quantities  of  this  constituent  that  are 
usually  present  in  rocks,  and  is  the  method  which  is  used  by  the 
chemists  of  the  U.  S.  Geological  Survey  and  that  which  I  also  use. 
It  will,  therefore,  be  described  first  in  detail,  while  the  gravimetric 
methods  will  be  taken  up  later,  as  they  are  useful  when  the  amount 
of  titanium  is  very  high,  and  all  analysts  may  not  have  at  hand  the 
appliances  necessary  for  the  colorimetric  method. 

Colorimetric  Method.1 — This  method,  which  was  suggested 
by  Weller,  depends  on  the  yellow  to  orange  coloration  of  titanium 
solutions  produced  by  hydrogen  peroxide,  the  depth  of  color  being 
proportional  to  the  amount  of  Ti(>2. 

Errors. — In  the  colorimetric  method  for  titanium  there  are 
few  sources  of  serious  error,  though  some  corrections  may  have  to 
be  applied. 

Vanadium,  molybdenum,  and  chromium  interfere,  the  first 
two  because  of  the  similar  coloration  of  their  solutions  by  hydrogen 
peroxide,  and  the  last  because  of  the  normal  color  of  the  chromates. 
It  is  very  seldom,  however,  that  any  of  these  elements  is  present 
in  rocks  in  sufficient  amount  to  affect  the  determination  seriously. 

1  Classen,  1,  pp.  776-778;  Hillebrand,  pp.  128-134;  Mellor,  pp.  85-86, 
203-206;  Treadwell,  2,  pp.  100-102;  H.  E.  Merwin,  Am.  Jour.  Sci.,  28,  pp. 
119-125,  1909. 


TITANIUM    DIOXIDE  169 

Hillebrand  has  shown  that  even  very  small  amounts  of  hydro- 
fluoric acid  "  render  this  method  inexact  by  partly  bleaching  the 
yellow  color."  The  hydrogen  peroxide  must,  therefore,  be  free 
from  fluorine,  as  also  must  be  the  solution  in  which  the  titanium 
is  determined.  This  bleaching  effect,  it  may  be  added,  has  been 
utilized  by  Steiger  and  Merwin  for  the  determination  of  fluorine 
(p.  235). 

Strong  solutions  of  ferric  sulphate  are  yellowish,  so  that  the 
color  of  this  salt  will  be  added  to  that  of  the  titanium,  and  the 
result  may  be  a  somewhat  serious  error  if  the  rock  contains  much 
iron.  This  may  be  corrected,  as  will  be  seen  later. 

Merwin  1  has  shown  that  alkali  sulphates  have  a  very  marked 
bleaching  effect.  As  his  standard  was  prepared  free  from  alkali 
sulphate,  there  can  be  no  doubt  as  to  the  reality  of  the  effect.  He 
shows,  however,  that  the  bleaching  is  diminished,  and  indeed 
rendered  almost,  if  not  wholly,  negligible,  if  sufficient  sulphuric 
acid  is  present. 

Dunnington  2  pointed  out  the  necessity  for  the  presence  of  at 
least  5  per  cent  of  sulphuric  acid,  but  his  explanation  that  this 
prevents  the  formation  of  metatitanic  acid,  which  does  not  become 
colored  by  hydrogen  peroxide,  is  probably  only  partially  correct, 
as  pointed  out  by  Merwin,  the  solutions  used  by  him  not  being 
strongly  enough  acid  to  prevent  the  bleaching  effect  of  the  alkali 
sulphate  present. 

Phosphoric  acid  has  also  a  bleaching  effect,  but  the  amount 
of  this  in  rocks  is  so  small  that  this  factor  is  negligible. 

The  eye  must  be  practiced  in  the  color  distinctions,  and  the 
illumination  must  not  affect  the  color  of  the  solution.  On  this 
account  the  determination  should  not  be  made  at  night. 

The  Operation. — The  preparation  of  the  standard  solution 
containing  0.01  gram  of  Ti02  in  10  c.c.  has  already  been  described 
(p.  55).  It  will  be  assumed  that  the  form  of  colorimeter  described 
on  p.  43  is  used.  If  Steiger's  or  Schreiner's  form  is  used,  such 
modifications  in  manipulation  are  to  be  observed  as  are  suggested 
in  Hillebrand 's  description.3 

The  process  is  as  follows:   The  solution  of  the  rock  in  which 

1  H.  E.  Merwin,  Am.  Jour.  Sci.,  28,  p.  119,  1909. 

2  F.  P.  Dunnington,  Jour.  Am.  Chem.  Soc.,  13,  p.  210,  1891. 

3  Hillebrand,  pp.  35-38. 


170  METHODS 

the  titanium  is  to  be  determined,  and  which  is  called  the  test  solu- 
tion, is  evaporated  down,  if  necessary,  to  an  appropriate  volume 
and,  when  cold,  is  placed  in  a  stoppered  measuring-flask  of  suit- 
able size.  The  basin  used  for  the  evaporation  must,  of  course,  be 
washed  several  times  into  the  flask.  At  least  10  c.c.  of  concen- 
trated sulphuric  acid  are  slowly  added,  and  the  liquid  is  well 
stirred.  When  it  is  cool  5  to  10  c.c.  of  hydrogen  peroxide,  or  more 
than  sufficient  to  oxidize  all  the  titanium,  is  added  and  the  liquid 
is  diluted  to  the  mark.  It  is  well  mixed,  not  by  shaking,  but  by 
inverting  the  closely  stoppered  flask  several  times,  the  stopper 
being  kept  in  place  by  the  finger. 

The  volume  to  which  the  solution  is  diluted  depends  on  the 
amount  of  titanium  dioxide  present  in  the  rock.  For  the  great 
majority  of  rocks,  in  which  there  is  a  little  less  than  1  per  cent, 
250  c.c.  is  suitable.  With  granites,  rhyolites,  and  such  rocks,  in 
which  the  percentage  of  titanium  dioxide  is  very  small,  a  volume 
of  100  per  cent  is  preferable.  With  rocks  that  contain  more  than 
1  per  cent  of  titanium  dioxide  the  volume  should  be  increased 
proportionately,  500  c.c.  being  appropriate  if  the  percentage  is 
about  2,  and  1000  c.c.  if  it  is  about  4  or  more.  It  is  essential  to 
have  the  depth  of  color  in  the  test  solution  less,  indeed,  consider- 
ably less,  than  that  of  the  standard  solution  diluted  as  described 
below. 

The  delicacy  of  this,  or  any  other,  colorimetric  method  is  at  a 
maximum  when  the  color  is  neither  very  deep,  nor  extremely  light, 
so  that,  when  much  titanium  is  present  the  dilution  should  be 
great.  The  most  favorable  tint  is  a  rather  deep  straw-color,  or 
about  that  of  light  beer,  or  clear,  light  amber.  This  corresponds 
almost  exactly  to  Ridgway's  "  light  cadmium."1  Stated  quan- 
titatively, a  favorable  strength  is  that  of  a  solution  that  contains 
from  0.00002  to  0.00004  gram  of  TiO2  per  c.c.;  in  other  words 
when  10  c.c.  of  the  diluted  standard  solution  described  below  has 
to  be  diluted  with  from  about  10  to  20  c.c.  of  water  to  bring  it  to 
the  same  tint  as  that  of  the  test  solution. 

To  proceed  with  the  operation :  an  indeterminate  quantity  of 
the  test  solution  is  poured  into  one  of  the  colorimeter  glasses,  say 
the  right-hand  one.  Ten  c.c.  of  the  standard  solution,  containing 
about  0.01  gram  of  TiO2  (the  quantity  being  known),  is  removed 

1  R.  Ridgway,  Color  Standards,  Plate  IV;  Washington,  1912. 


TITANIUM   DIOXIDE  171 

from  the  stock  bottle  with  a  dry  10-c.c.  pipette  x  and  placed  in  a 
100-c.c  stoppered  measuring  flask  Five  c.c.  of  hydrogen  peroxide 
are  added,  and  the  liquid  is  diluted  with  water  to  the  mark  and 
well  mixed.  Each  cubic  centimeter  of  this  diluted  standard  will 
then  contain  0.0001  gram  of  TiO2.  This  amount  of  diluted 
standard  will  suffice  for  the  determinations  in  three  rocks.  The 
color  disappears  after  a  time,  so  the  diluted  standard  must  be 
made  up  fresh  for  each  determination  or  batch  of  determinations. 
It  is  evident  that  the  color  cannot  be  restored  by  addition  of 
hydrogen  dioxide  to  a  solution  already  diluted  to  the  mark,  as 
this  will  increase  the  volume  of  liquid  and  so  lessen  the  amount 
of  Ti02  per  cubic  centimeter.  If  a  series  of  rock  analyses  is  being 
made,  it  is  well,  in  order  to  economize  the  standard  solution,  to 
make  the  titanium  determinations  in  batches  of  three  rocks  at 
a  time,  the  earlier  test  solutions  being  kept  in  suitable  flasks,  but 
not  diluted  to  the  mark,  until  the  determinations  are  to  be  made. 

Two  burettes  are  fixed  in  a  stand;  the  one  is  filled  with  the 
diluted  standard,  and  the  other  with  water,  the  position  of  the 
meniscus  in  each  being  noted.  Ten  c.c.  of  the  diluted  standard 
are  then  run  into  the  left-hand  glass,  and  water  added  from  the 
other  burette,  in  small  quantities  at  a  time.  After  each  addition 
the  liquid  is  mixed  by  a  gentle  rotary  motion  of  the  glass,2  and  the 
color,  or  rather  the  degree  of  dilution  of  the  color,  is  compared 
with  that  of  the  test  solution.  In  doing  this  the  shutter  should  be 
slid  down  till  only  the  liquid  in  each  glass  is  visible,  and  none  of  the 
empty  space  above.  As  the  color  of  the  diluted  standard  ap- 
proaches that  of  the  test  solution,  the  addition  of  water  should 
be  cautious  and  by  a  few  drops  at  a  time,  till  the  point  of  agree- 
ment is  reached,  when  the  amount  of  water  added  is  read  off 
and  noted  down.  The  liquid  is  rejected. 

Ten  c.c.  of  the  diluted  standard  are  then  again  run  into  the 
again  empty  left-hand  glass  and  water  is  added  as  before,  the 
water  burette  being  refilled  if  it  is  necessary.  This  procedure  is 
repeated  a  third  time,  so  that  there  is  obtained  a  mean  of  three 
determinations,  which  should  not  vary  more  than  within  1  c.c. 

1  The  solution  should  not  be  poured  out,  and  the  bottle  is  to  be  closed  as 
soon  as  the  portion  is  removed. 

2  A  glass  rod  flattened  at  one  end  (Hillebrand)  may  be  used  but  there  is 
danger  of  breaking  the  glass. 


172  METHODS 

In  adding  the  water  the  second  and  third  times  it  is  well  to  cover 
the  burette  with  a  roll  of  paper  held  in  place  by  an  elastic  band, 
so  as  to  avoid  any  bias  produced  by  a  knowledge  of  the  amount  of 
water  that  is  to  be  added  to  make  the  second  and  third  observa- 
tions agree  with  the  first. 

While  observing  the  color  after  each  addition  of  water,  the  box 
is  held  in  the  hand  with  the  ground-glass  end  pointed  toward  a 
good,  natural  light,  that  of  light  clouds,  if  possible,  or  the  window. 
The  disturbing  effect  of  sunlit  foliage  or  brick  walls  is  to  be  avoided, 
and,  if  necessary,  a  towel  is  pinned  against  the  window  so  as  to 
furnish  a  white  screen. 

The  operation  should  be  carried  out  in  the  day  time,  best  on  a 
bright  day,  as  the  shades  of  color  are  much  more  readily  distin- 
guishable by  daylight  than  by  artificial  light.  It  will  be  found 
advantageous  to  rest  the  eyes  occasionally  by  looking  at  the  floor 
or  at  a  dark  corner,  as  their  sensitiveness  is  apt  to  diminishjwith 
fatigue. 

When  testing  the  method  with'  known  amounts  of  Ti(>2  for 
the  first  few  times  I  noticed  a  tendency  to  judge  that  the  colors 
matched  some  time  before  they  actually  should  have  done  so. 
Any  such  tendency,  or  the  reverse,  which  may  be  true  of  others,1 
is  to  be  guarded  against;  to  do  this  one  must  have  practice  with 
known  amounts  of  titanium.  This  may  be  obtained  by  making 
up  test  solutions  from  small  measured  volumes  of  standard  solu- 
tion diluted  with  varying  known  volumes  of  water,  and  deter- 
mining the  Ti02  in  them.  As  the  amount  of  TiO2  is  known,  one 
has  a  check  on  the  personal  equation,  and  will  soon  be  in  a  posi- 
tion to  determine  unknown  quantities  of  Ti(>2.  For  one  who  has 
never  used  the  method,  this  preliminary  practice  should  not  be 
omitted.  After  a  little  practice  one  soon  becomes  able  to  judge 
of  exact  agreement  in  color  and  to  arrive  at  concordant  and 
correct  results. 

The  principle  underlying  the  simple  calculation  that  is  needed 
to  determine  the  percentage  of  titanium  dioxide  is  that,  as  the 
colors  in  the  two  solutions  are  identical,  the  amount  of  TiC>2 
per  cubic  centimeter  is  the  same  in  both.  An  example,  taken  from 
the  analysis  presented  on  p.  242,  is  given  here,  so  that  the  prin- 
ciple and  the  calculations  may  be  clearly  understood.  They 
1  Mellor,  pp.  85-86,  Merwin,  Am.  Jour.  Sci.,  28,  p.  120,  1909. 


TITANIUM   DIOXIDE  173 

apply  as  well  to  the  colorimetric  determinations  of  manganese  and 
chromium. 

The  portion  of  rock  powder  taken  for  the  main  fusion  weighed 
1.0197  gram.  The  test  solution,  after  the  determination  of  total 
iron  oxides,  was  made  up  to  500  c.c.  It  was  found  that  12.11  c.c.1 
of  water  had  to  be  added  to  10  c.c.  of  tenth  diluted  standard,  which 
contained  0.0074  gram  2  of  TiO2  per  cubic  centimeter,  to  make  its 
color  match  that  of  the  test  solution.  We  now  have  to  divide 
0.00074  (the  weight  of  TiO2  in  1  c.c.  of  the  diluted  standard)  by 
10+12.11  =  22.11  (the  volume  of  identically  colored  standard)  to 
obtain  the  weight  of  Ti02  per  cubic  centimeter.  The  result  is 
0.000033469  gram.  This  multiplied  by  500  (the  volume  of  the 
test  solution)  gives  0.0167345  gram  as  the  amount  of  TiO2  in  the 
rock.  Divided  by  1.0197  this  gives  1.64  as  the  percentage  of  TiO2 
in  the  rock. 

A  correction  for  the  disturbing  effect  of  high  iron  may  be  made 
by  allowing  for  this  on  the  basis  of  Hillebrand's  3  tests,  that  "  go 
to  show  that  the  coloring  effect  of  0.1  gram  Fe2C>3  in  100  c.c.  of 
5  per  cent  sulphuric  acid  solution  is  about  equal  to  0.2  milligram 
of  Ti02  in  "100  c.c.  when  oxidized  by  hydrogen  peroxide.  This 
amounts  to  a  correction  of  only  0.02  per  cent  on  1  gram  of  rock 
containing  the  unusual  amount  of  10  per  cent  Fe20s."  As  phos- 
phoric acid  bleaches  ferric  sulphate  solutions  as  well  as  those  of 
oxidized  titanium,  it  can  be  used  to  equalize  the  bleaching  in  the 
test  and  standard  solutions  by  adding  a  known  amount  of  phos- 
phoric acid  or  a  soluble  phosphate  to  each.  As  this  will  rarely  be 
called  for,  the  student  would  best  consult  Hillebrand  (just  cited) 
for  the  details. 

In  regard  to  the  correction  for  the  effect  of  alkali  sulphates  I 
cannot  do  better  than  quote  Merwin4  verbatim. 

"  In  rock  analysis  by  using  6  grams  of  pyrosulphate,  which 
is  equivalent  to  4  grams  of  normal  sulphate  and  2  grams  of  acid, 

1  This  is  the  mean  of  11.8,  12.5;  and  12.1  c.c. 

2  Some  titanium  oxide  had  precipitated  on  standing,  and  the  solution  had 
been  filtered  and  restandardized  by  determining  gravimetrically  the  amount 
of  TiO2  per   cubic  centimeter  (p.  55).     It  will  be  clear  that  the   standard 
does  not  need  to  contain  exactly  0.01  gram  TiO2  per  cubic  centimeter. 

3  Hillebrand,  Bull.  422,  p.  133. 

4  H.  E  Merwin,  Am.  Jour.  Sci.,  28,  p.  122,  1909. 


174  METHODS 

for  the  melt  containing  the  titanium,  and  dissolving  this  in  water 
to  which  10  c.c.  of  strong  sulphuric  acid  has  been  added,  a  nearly 
negligible  correction  of  only  3  per  cent  (of  the  TiO2)  need  be  added. 
If  the  Ti02  exceeds  .02  gram  no  correction  is  required  (as  the 
color  is  so  intense  that  the  bleaching  effect  will  not  be  noticeable). 
In  case  the  melt  is  dissolved  in  100  c.c.  of  5  per  cent  sulphuric  acid, 
the  titanium  found — if  the  amount  is  between  .002  gram  and  .01 
gram — is  too  low  by  approximately  .0004  gram."  It  will  be 
seen  that  for  most  work  the  correction  is  negligible  if  the  solution 
is  made  very  strongly  acid. 

If  the  glasses  described  above  are  not  available,  and  it  is  desired 
to  use  Nessler  tubes,  the  method  is  modified  as  follows,  according 
to  the  plan  of  Prof.  Penfield.  A  light  box  is  used  of  such  dimen- 
sions as  to  snugly  hold  the  two  tubes  side  by  side.  These  rest 
either  on  a  ground-glass  plate  forming  a  false  bottom,  or  on  a 
horizontal  wooden  partition  with  holes  or  a  broad  slot  cut  so  as 
to  admit  light  from  below.  Beneath  this  or  the  ground-glass 
plate  a  mirror  is  fixed  at  an  angle  of  45°  above  the  real  bottom, 
admitting  light  from  a  side-opening  and  transmitting  it  vertically 
up  through  the  tubes. 

The  test  solution  is  prepared  as  above,  but  the  standard  is 
used  undiluted.  One  Nessler  tube  is  filled  with  the  colored  test 
solution  up  to  the  50-c.c.  mark,  and  in  the  other  is  placed  5  c.c. 
of  hydrogen  peroxide,  which  is  diluted  with  a  known  volume  of 
water  nearly  up  to  the  same  mark.  The  standard  solution  is 
then  added  in  very  small  quantities  at  a  time  from  a  burette,  the 
liquid  being  stirred,  and  the  colors  observed  after  each  addition, 
till  there  is  agreement  between  the  two.  After  a  few  trials,  and 
with  knowledge  of  the  approximate  amount  of  titanium  present 
in  the  rock,  the  heights  of  the  two  solutions  can  be  made  sensibly 
identical,  but  several  determinations  are  always  advisable.  The 
Nessler  tube  for  the  standard  solution  is  to  be  emptied  and  washed 
carefully  each  time. 

While  this  modification  involves  the  use  of  more  easily  obtain- 
able glass  vessels,  as  well  as  less  standard  solution,  it  is  not  quite 
as  accurate  as  the  other,  although  sufficiently  so  for  many  pur- 
poses, and  is  far  more  so  than  the  gravimetric  method  sometimes 
used. 

Another  alternative  apparatus  is  that  of  Schreiner,  described 


TITANIUM  DIOXIDE  175 

by  Hillebrand.1  This  consists  essentially  of  two  graduated 
glass  tubes  to  hold  the  test  and  diluted  standard  solutions,  the 
liquids  being  examined  vertically.  The  depths  of  the  two  col- 
umns of  liquid  are  changed  by  means  of  two  smaller  glass  tubes 
with  flat  bottoms  immersed  in  them,  the  graduated  tubes  being 
moved  up  or  down  until  the  shades  are  identical.  The  strengths 
of  the  two  solutions  are  inversely  as  the  heights  of  the  columns. 
This  apparatus  is  said  to  give  good  results.  A  new  form  of  color- 
imeter which  has  recently  been  described  by  G.  Steiger  2  utilizes 
the  same  principle.  It  is  the  form  which  is  now  used  in  the  Survey 
laboratory. 

Gravimetric  Methods. — Although  the  colorimetric  method  for 
the  determination  of  titanium  is  by  far  the  simplest  and  most  expe- 
ditious, is  capable  of  adequate  accuracy,  and  is  applicable  in  the 
great  majority  of  rocks,  it  is  not  so  well  suited  to  rocks  or  minerals 
which  contain  more  than  about  5  per  cent  of  Ti02,  because  of  the 
loss  in  delicacy  with  great  depth  of  color,  and  the  possibility  of 
serious  error  with  the  large  dilution  necessary  to  overcome  this. 
Occasion  may  therefore  arise  for  the  determination  of  titanium  by 
gravimetric  methods.  Although  the  use  of  these  is  not  recom- 
mended, if  the  colorimetric  method  is  applicable,  some  of  the 
recent  and  more  accurate  methods  may  be  described  briefly. 

One  of  the  best  gravimetric  methods  is  that  of  Gooch,3  which  is 
fully  described  by  Hillebrand,4  to  whom  the  student  may  be 
referred.  Although  it  is  rather  complicated,  it  is  satisfactorily 
accurate  for  most  rocks.  Hillebrand  has  shown  that  it  is  not  to 
be  used  when  zirconia  is  present,  but  as  large  amounts  of  these  two 
oxides  rarely  occur  together  in  the  same  rock,  this  consideration 
is  of  very  slight  practical  importance. 

A  simplified  modification  of  this  method,  due  to  Prof.  H.  Fay, 
is  described  by  Warren,5  who  states  that  it  is  "  highly  satisfactory, 
both  in  point  of  simplicity  and  accuracy."  His  description  of  it 
is  quoted  here. 

"  Fuse  0.4-0.6  gram  of  finely  ground  ore  with  6-8  times 

1  Hillebrand,  p.  37. 

2  jbid.,  p.  35. 

3  F.  A.  Gooch,  U.  S.  Geol.  Surv.,  Bull.  27,  p.  16,  1886. 

4  Hillebrand,  p.  134. 

5  C.  H.  Warren,  Am.  Jour.  Sci.,  25,  p.  23,  1908. 


176  METHODS 

its  weight  of  mixed  alkali  carbonates  until  action  ceases.  Extract 
the  mass  with  hot  water,  and  decant  the  solution  through  a 
filter.  Boil  the  residue  with  25  c.c.  of  sodium  carbonate  solu- 
tion, filter  and  then  wash  the  residue  on  the  filter  paper  several 
times  with  dilute  sodium  carbonate  solution.  Place  the  filter 
and  residue  in  a  platinum  crucible  and  ignite  at  a  low  temper- 
ature until  the  filter  paper  is  burned.  Fuse  with  12-15  parts 
(5-9  grams)  of  dry  acid  potassium  sulphate  for  one-half  hour. 
The  temperature  of  the  fusion  should  be  so  regulated  that  the 
mass  is  kept  in  the  molten  condition,  but  sulphur  trioxide  should 
escape  only  when  the  lid  of  the  crucible  is  removed.  Cool  and 
remove  the  fusion  from  the  crucible  by  means  of  a  long  platinum 
wire,  which  has  been  inserted  in  the  fused  mass.  Suspend  the 
fusion  in  200  c.c.  of  cold  water  to  which  has  been  added  100  c.c. 
of  sulphurous  acid  and  allow  to  stand  in  a  cool  place  until  solution 
is  complete. 

"  Filter  if  necessary.  To  the  solution  add  125  c.c.  acetic 
acid  (sp.gr.  1.04)  and  dilute  to  800  c!c.  in  a  liter  beaker.  Add 
20  grams  of  sodium  acetate  dissolved  in  a  small  amount  of  water 
and  boil  from  3-5  minutes,  adding  just  before  the  boiling-point  is 
reached  an  additional  25  c.c.  of  sulphurous  acid.  Allow  to  stand 
in  a  warm  place  for  one-half  hour  and  then  filter  by  means  of  a 
siphon  through  a  9-cm.  paper. 

"  Wash  the  precipitate  with  5  per  cent  acetic  acid  solution 
until  most  of  the  sulphate  has  been  removed,  and  then  ignite 
the  paper  and  precipitate.  Fuse  with  acid  potassium  sulphate 
again.  Proceed  exactly  as  before,  finally  igniting  and  weighing 
the  precipitate  as  Ti02." 

Warren  states  that  in  two  determinations  0.01  per  cent  of  iron 
could  be  detected  in  the  precipitate  and  that  probably  the  same 
amount  of  alumina  and  manganese  are  included.  For  very  accu- 
rate work  a  third  precipitation  is  advisable.  In  a  letter  he  lays 
stress  on  the  importance  of  adding  the  additional  25  c.c.  of  SO2 
just  before  the  boiling  begins. 

Another  modification  of  Gooch's  method  has  been  worked  out 

by  Thornton  1  in  analyzing  some  rocks  that  are  exceptionally  high 

in  iron  and  titanium.     It  depends  on  the  reduction  of  the  iron  in 

presence  of  tartaric  acid  (to  hold  up  the  titanium)  and  precipita- 

1  W.  M.  Thornton,  Jr.,  Am.  Jour.  Sci.,  34,  pp.  173,  214,  1912. 


LIME   (AND  STRONTIA)  177 

tion  of  the  iron  as  sulphide.  After  oxidation  and  destruction  of 
the  tartaric  acid,  by  the  action  of  sulphuric  and  nitric  acids,  the 
titanium  is  determined  by  the  basic  acetate  method.  The  pub- 
lished results  are  very  satisfactory. 

As  I  have  tried  neither  of  these  methods,  they  can  but  be  sug- 
gested to  the  student  for  use  in  case  of  need. 
|&s  Thornton  has  also  J  devised  a  method  for  the  separation  of 
titanium  from  iron,  aluminum,  and  phosphoric  acid  by  the  use  of 
"  cupferron."  This  method  seems  to  be  promising,  and  if  the 
reagent  needed  becomes  readily  procurable,  will  probably  be 
worthy  of  trial  in  special  cases. 

The  method  of  precipitating  metatitanic  acid  by  prolonged 
boiling  of  a  very  dilute  sulphuric  acid  solution  in  the  presence  of 
sulphur  dioxide  should  not  be  used,  as  this  antiquated  method  is 
very  unreliable.  Precipitation  of  metatitanic  acid  is  by  no  means 
complete  in  all  cases,  and  that  which  is  precipitated  is  almost 
always  contaminated  by  alumina  and  ferric  oxide.  It  is  also 
extremely  liable  to  adhere  very  firmly  to  the  sides  of  the  beaker, 
from  which  it  is  removed  with  great  difficulty.  It  is  as  apt  to 
give  too  high  as  too  low  results,  and,  after  thorough  trial,  with 
various  modifications,  I  have  rejected  this  method  entirely. 

10.  LiME2  (AND  STRONTIA) 

For  the  determination  of  lime  the  filtrate  from  the  ammonia 
precipitations  is  used  (p.  157);  or  if  manganese  has  been  deter- 
mined as  sulphide  (p.  224),  the  filtrate  from  this  may  be  used,  in 
which  case  the  ammonium  sulphide  need  not  be  destroyed.  The 
volume  of  liquid  should  not  amount  to  more  than  about  500  c.c., 
and  is  contained  in  an  800-c.c.  beaker.  If  it  is  more  than  this  it  is 
advisable  to  evaporate  it  down  (in  a  porcelain  or  platinum  basin) 
to  400  c.c.  This  should  not  be  necessary  if  care  has  been  taken 
to  avoid  unduly  large  quantities  of  wash  water.  If  a  precipitate 
(of  calcium  carbonate)  forms  it  is  dissolved  with  a  little  acid. 

Errors. — There  are  very  few  possible  errors  involved  in  the 

1  W.  M.  Thornton,  Jr.,  Am.  Jour.  Sci.,  37,  p.  407,  1914. 

2  Classen,  1,  p.  794;  Fresenius,  1,  pp.  270-272;  Gooch,  pp.  86-88;  Hille- 
brand,  pp.  118-119;    Mellor,  pp.  211-214;   Morse,  pp.  430-431;   Treadwell, 
2,  pp.  70-71. 


178  METHODS 

determination  of  lime;  indeed,  as  has  been  said  already,  this  con- 
stituent and  silica  are  those  which  are  most  likely  to  be  approx- 
imately correct  in  second-rate  work. 

The  error  caused  by  the  presence  of  carbonate  in  the  ammonia 
water  has  been  mentioned  on  p.  148,  in  connection  with  the  deter- 
mination of  alumina.  This,  of  course,  would  render  the  apparent 
amount  of  lime  too  low,  but  it  can  be  prevented  very  easily  by 
testing  the  ammonia  water  with  calcium  or  barium  chloride.  The 
magnitude  of  this  error  will  never  be  very  great,  unless  grossly 
impure  or  very  old  ammonia  water  is  used. 

The  first  precipitate  of  calcium  oxalate  invariably  contains 
some  magnesia  and  soda,  so  that  solution  and  reprecipitation  are 
always  called  for.  The  oxalate  should  be  ignited  to  oxide,  never 
weighed  as  carbonate  or  sulphate,  and  the  oxide  should  be  ignited 
to  constant  weight,  at  least  until  experience  has  taught  the  time 
necessary  for  the  ignition.  Blasting  is  not  necessary. 

Precipitation. — About  25  c.c.  of  ammonia  water  (previously 
tested  for  carbonate)  is  added,  and  I  have  found  it  useful  also  to 
add  50  c.c.  of  alcohol.  The  liquid  is  stirred  and  heated  to  boiling. 
During  the  heating  1-3  grams  of  ammonium  oxalate,  according 
to  the  amount  of  lime  in  the  rock,  are  dissolved  in  25  c.c.  of  water 
with  the  aid  of  heat.  This  solution  is  poured  into  the  large  beaker 
when  the  liquid  in  it  begins  to  boil  and  the  whole  is  well  stirred. 
The  boiling  is  continued  for  a  few  minutes,  and  the  liquid  is  allowed 
to  stand  for  two  or  three  hours,  or  better  for  six,  or  over  night,  if 
there  is  but  little  lime. 

The  liquid  is  filtered,  after  standing,  through  a  7-  or  9-cm. 
filter,  according  to  the  amount  of  lime,  the  filtration  and  first 
washing  being  carried  out  as  described  above  (p.  92).  The  filtrate 
is  received  in  an  800-c.c.  beaker,  and  when  this  becomes  rather 
more  than  half  full,  another  800-c.c.  beaker  is  substituted  for  it. 
This  is  rather  more  convenient  than  using  one  1000-c.c.  beaker. 

After  slight  washing,  the  precipitate  in  the  filter  is  dissolved 
and  washed  into  the  original  800-c.c.  beaker  as  has  been  described 
(p.  94),  a  few  cubic  centimeters  of  ammonium  oxalate  solution 
are  added  to  the  filtrate,  the  liquid  is  heated  nearly  to  boiling,  and 
the  lime  reprecipitated  by  the  addition  of  sufficient  ammonia 
water  to  give  a  strong  ammoniacal  odor.  A  little  alcohol  may 
also  be  added. 


LIME   (AND  STRONTIA)  179 

/ 

After  standing  for  several  hours  the  precipitate  is  filtered  off, 

the  filtrate  passing  into  the  second  800-c.c.  beaker.     The  calcium 

oxalate  is  washed  well,  but  not  over-washed,  some  drops  of  the 

•  ammonium  oxalate  solution  being  added  with  each  addition  of 

wash  water.     A  second  reprecipitation  is  not  called  for. 

The  precipitate  is  ignited  moist  as  already  described  (p.  102). 
The  calcium  oxide  is  ignited  over  a  Meker  burner  for  thirty  min- 
utes, which  will  be  sufficient  to  change  the  oxalate  completely  to 
oxide  in  most  rocks.  If  much  lime  be  present  it  is  well  to  ignite 
again,  after  weighing,  until  the  weight  is  constant.  The  calcium 
oxide  should  be  kept  covered  in  the  desiccator,  and  weighed  as 
rapidly  as  possible,  with  the  crucible  covered.  Its  weight  is 
divided  by  that  of  the  rock  powder  taken,  in  order  to  get  the  per- 
centage of  CaO. 

Strontia. — The  calcium  oxide,  as  obtained  above,  contains 
all  the  strontia  of  the  rock,  but  scarcely  ever  more  than  traces  of 
the  baryta.  The  amount  of  strontia  is  always  very  small,  not  more 
than  1  milligram  or  less,  except  in  a  very  few,  highly  unusual 
rocks.  In  most  analyses,  therefore,  it  need  not  be  determined, 
especially  as  it  enters  rock  minerals  as  an  isomorphous  replacer  of 
lime.  It  will  be  well,  however,  to  determine  it  in  some  rocks,  as 
in  one  or  two  of  a  series,  since  we  know  little  of  its  relationships 
to  the  other  constituents.  There  are  no  serious  errors  involved. 

The  method  that  is  recommended  is  based  on  the  solubility  of 
calcium  nitrate  in  a  mixture  of  ether  and  absolute  alcohol,  and  the 
insolubility  of  strontium  nitrate  in  this  medium.1 

After  the  lime  has  been  finally  weighed,  and  before  it  has 
absorbed  any  appreciable  amount  of  carbon  dioxide  from  the  air, 
it  is  slightly  more  than  moistened  with  a  few  drops  of  water  in  the 
crucible  in  which  it  was  weighed.  Nitric  acid  is  added,  drop  by 
drop,  until  the  lime  is  just  dissolved.  The  contents  of  the  crucible 
are  evaporated  to  complete  dry  ness  on  the  water-bath,  and  when 
cold  about  5  c.c.  of  a  mixture  of  absolute  alcohol  and  ether  is 
poured  in.  The  crucible  is  tightly  closed  with  a  well-fitting  cork, 
and  laid  aside  in  a  cool  place  for  at  least  twenty-four  hours. 

The  contents  of  the  crucible  are  then  to  be  filtered  through  a  Si- 
cm,  filter  and  well  washed  (six  times)  with  the  same  mixture  of 

1  Classen,  1,  p.  797;  Fresenius,  1,  p.  621;  Hillebrand,  pp.  119-120;  Mellor, 
pp.  514-515;  Treadwell,  2,  pp.  78-80. 


180  METHODS 

absolute  alcohol  and  ether.  The  filter  is  allowed  to  dry  in  the 
funnel,  after  which  the  strontium  nitrate  is  dissolved  in  a  few 
cubic  centimeters  of  water  passed  through  the  filter  and  received 
in  a  50-c.c.  beaker,  the  filter  being  washed  a  few  times.  A  few 
drops  of  dilute  sulphuric  acid  are  added  and  then  alcohol  equal  in 
amount  to  the  volume  of  liquid  in  the  beaker.  After  standing 
for  twelve  to  twenty-four  hours  the  precipitated  strontium  sul- 
phate is  filtered  off,  ignited  and  weighed.  Its  weight  is  multi- 
plied by  .56  to  obtain  that  of  SrO,  and  this  is  deducted  from  that 
of  the  lime. 

11.  MAGNESIA  J 

The  filtrate  from  the  calcium  oxalate  contains,  of  the  original 
rock  constituents,  only  the  magnesia  and  alkalies,  with  the  barium, 
and  part  of  the  manganese  and  the  nickel  and  other  metals  of  the 
sulphide  group,  if  these  have  not  been  previously  separated. 
There  are,  of  course,  also  present  the  alkalies  derived  from  the 
carbonate  fusion  and  large  amounts  of  ammonium  salts.  It  will 
not  be  necessary  to  remove  these  last  for  the  determination  of  the 
magnesia,  which  is  the  only  constituent  that  interests  us  in  this 
filtrate. 

Errors. — The  chief  source  of  error  in  the  determination  of 
magnesia  is  that  already  mentioned  in  connection  with  alumina, 
namely,  the  tendency  to  partial  precipitation  by  ammonia  along 
with  alumina.  This  must  be  prevented  by  the  presence  of  suffi- 
cient ammonium  salts  and  repeated  precipitations,  as  already 
described. 

An  error  of  less  magnitude  and  importance,  but  which  should 
be  avoided,  is  that  involved  in  the  precipitation  of  the  ammonium- 
magnesium  phosphate.  If  there  be  present  excess  of  ammonia, 
ammonium  salts  and  precipitant,  the  ammonium-magnesium 
phosphate,  and  hence  the  magnesium  pyrophosphate,  will  not  be 
normal  in  composition,  owing  to  the  presence  of  extra  P20s,  as 
pointed  out  by  Neubauer  2  and  by  Gooch  and  Austin.3  This  must 

1  Classen,  1,  pp.  830-834;   Fresenius,  1,  pp.  275-276;   Gooch,  pp.  81-84; 
Hillebrand,  pp.  123-126;   Mellor,  pp.  215-221;  Ostwald,  pp.  149-151;  Stieg- 
litz,  p.  165;  Treadwell,  2,  pp.  65-67. 

2  Neubauer,  Z.  Angew.  Chem.,  1896,  p.  435. 

3  Gooch  and  Austin,  Am.  Jour.  Sci.,  7,  p.  187,  1899. 


MAGNESIA  181 

be  corrected  by  solution  of  the  first  precipitate  and  reprecipitation 
from  the  acid  solution  by  a  slight  excess  of  ammonia.  This  error 
will  not  affect  the  other  constituents,  but  will  raise  the  figures  for 
MgO  only,  and  hence  the  summation  of  the  analysis  will  be  too 
high.  The  error  due  to  this  cause  will  usually  be  of  slight  magni- 
tude and  seldom  of  the  importance  attributed  to  it  by  Robinson.1 

The  magnesia  will  be  low  if  the  precipitate  of  calcium  oxalate 
is  not  dissolved  and  reprecipitated. 

A  marked  loss  has  been  shown  by  Connor2  to  take  place  on 
evaporation  of  the  magnesium  pyrophosphate  with  nitric  acid,  as 
is  often  done  to  free  it  from  adherent  traces  of  carbon. 

The  figure  for  magnesia  will  also  be  slightly  high,  if  manganese 
has  not  been  separated  previously.  For  most  rocks  this  error  is 
quite  negligible. 

Precipitation. — The  total  filtrate  containing  all  the  magnesia 
now  amounts  to  some  800  c.c.  or  more,  contained  in  two  800-c.c. 
beakers.  To  each  is  added  100  c.c.  of  ammonia  water  and  about 
50  c.c.  of  alcohol.  A  solution  is  made  of  about  3  grams  of  sodium- 
ammonium  phosphate  in  50  c.c.  of  hot  water  and,  after  stirring, 
this  is  divided  between  the  two  beakers.  The  precipitation  is 
made  in  the  cold,  not  in  a  hot  solution.  The  mixture  in  each 
beaker  is  to  be  well  stirred,  taking  the  precaution  not  to  let  the 
stirring-rod  touch  the  sides,  and  the  beakers  are  laid  aside  (cov- 
ered) for  at  least  twelve,  and  preferably  twenty-four  hours. 

At  the  end  of  this  time  the  liquid  is  filtered  and  the  beaker  and 
the  precipitate  are  washed  slightly  with  water  containing  about 
5  per  cent  of  ammonia  water,  the  filtrate  being  rejected.  The 
precipitate  in  the  filter  is  then  dissolved  in  about  25  c.c.  of  warm, 
dilute  nitric  acid  (1  :  5),  the  solution  being  caught  in  one  of  the 
two  800-c.c.  beakers.  The  precipitate  adhering  to  the  sides  of 
both  is  also  washed  down  and  dissolved  in  dilute  nitric  acid,  which 
is  passed  through  the  filter,  and  the  beakers  and  filter  are  well 
washed.  All  this  is  carried  out  as  described  on  pp.  91-98. 

At  the  end,  the  nitric  acid  solution,  which  will  be  in  but  one  of 
the  beakers  and  should  amount  to  not  more  than  150-200  c.c., 
will  contain  all  the  magnesia.  About  one-third  of  this  volume  of 
ammonia  water  is  added,  as  well  as  a  little  alcohol,  and  one  or  two 

1  H.  H.  Robinson,  Am.  Jour.  Sci.,  41,  p.  260,  1916. 

2  M.  F.  Connor,  12  Cong.  Geol.  Int.,  C.  R.,  p.  886,  1914. 


182  METHODS 

drops  of  the  phosphate  solution,  the  liquid  is  stirred,  and  again 
allowed  to  stand.  Three  or  four  hours  of  standing  will  be  sufficient. 

The  final  precipitate  may  be  collected  in  either  a  Gooch  crucible 
or  a  paper  filter.  The  former  is  rather  the  better,  on  the  whole, 
though  it  makes  little  difference  which  is  used.  If  a  Gooch  cru- 
cible is  used,  this  is  prepared  and  the  filtration  and  ignition  are 
carried  out,  as  described  on  pp.  99-101.  If  paper  is  used  the 
precipitate  is  collected  on  the  same  filter  that  has  been  used  before, 
a  little  weak  ammonia  being  passed  through  it  first.  As  the  fine 
precipitate  has  a  tendency  to  creep  up  the  glass  wall  of  the  funnel, 
this  should  be  guarded  against.  The  ignition  in  the  moist  paper 
is  carried  out  as  with  the  silica,  except  that  the  paper  should  be 
carbonized  and  burnt  away  more  slowly. 

In  either  case  the  precipitate  is  ignited  over  a  Meker  burner 
for  twenty  minutes  or  more,  and  brought  to  constant  weight. 
Blasting  is  not  generally  necessary. 

After  cooling  in  the  desiccator  the  crucible  and  its  contents 
are  weighed,  the  gain  in  weight  being  Mg2?2O7.  This  is  to  be 
multiplied  by  the  factor  0.3621  to  reduce  it  to  MgO. 

Beginners  will  do  well  to  test  the  completeness  of  the  separation 
of  magnesia  and  alumina  by  examining  the  solution  in  which  TiO2 
has  been  determined  (p.  169).  Alumina  and  iron  are  to  be  pre- 
cipitated by  ammonia  in  the  whole,  or  an  aliquot  part  of  this 
solution.  The  precipitate  is  filtered  off,  and  magnesia,  if  present, 
is  determined  by  the  method  just  described. 

12.  FERROUS  OXIDE  1 

Ferrous  oxide  is  determined  in  a  separate  portion  of  rock  pow- 
der. For  rocks,  such  as  granites  or  rhyolites,  that  contain  but 
little  of  this  constituent,  it  is  well  to  use  1  gram;  but  for  most  rocks 
\  gram  will  be  an  appropriate  amount. 

Several  methods  of  different  kinds  have  been  proposed,  and 
will  be  found  mentioned  by  the  authorities  just  cited.  Of  these, 
that  of  Mitscherlich 2  has  been  frequently  used,  especially  in 
Europe.  It  consists  in  decomposing  the  rock  powder  with  dilute 

1  Classen,  2,  pp.  620-623;   Hillebrand,  pp.  154-171;   Mellor,  pp.  461-466; 
Treadwell,  2,  pp.  502-504. 

2  For  a  discussion  of  this  method  see  Hillebrand,  p.  157. 


FERROUS  OXIDE  183 

sulphuric  acid  in  a  sealed  glass  tube  under  pressure.  This  method 
is  open  to  many  objections:  liability  of  the  glass  to  be  attacked; 
difficulty  of  ascertaining  when  decomposition  is  complete;  the 
insuring  of  an  oxygen-free  atmosphere;  the  inaccuracy  of  the 
method  if  sulphides  are  present;  and  the  great  trouble  and  time 
involved  in  the  procedure.  The  Mitscherlich  method  will, 
therefore,  not  be  considered  further. 

There  are  several  modifications  of  the  method  of  decomposing 
the  rock  powder  by  a  mixture  of  sulphuric  and  hydrofluoric  acids 
in  a  non-oxidizing  atmosphere.  Of  these  those  of  Cooke  and  of 
Treadwell  are  often  recommended,  but  as  they  demand  somewhat 
elaborate  apparatus,  do  not  readily  insure  complete  decomposition, 
and  take  much  time,  they  also  will  not  be  described  here.1 

The  only  method  that  will  be  dealt  with  here  is  Pratt's  simpli- 
fied modification,2  which  has  been  used  for  years  and  found  to  give 
concordant  and  satisfactory  results. 

Errors. — The  correct  determination  of  ferrous  oxide  has  long 
been  one  of  the  most  difficult  and  uncertain  points  in  the  analysis 
of  rocks.  The  chief  difficulty,  of  course,  lies  in  the  solution  of  the 
material  without  oxidation  of  the  ferrous  oxide  to  ferric. 

It  was  formerly  thought  to  be  imperative  that,  whatever  the 
method  employed  for  the  determination  of  ferrous  oxide,  the  rock 
powder  should  be  in  an  extremely  fine  state  of  division,  and  the 
use  of  a  specially  ground  powder  was  previously  advocated  by 
Hillebrand  and  also  by  myself  in  the  first  edition  of  this  book. 

Mauzelius3  has,  however,  shown  that  there  is  a  decided  ten- 
dency to  oxidation  of  the  ferrous  iron  during  grinding,  and  that  this 
becomes  the  more  marke'd  the  longer  the  grinding  and  the  finer 
the  powder,  so  that  he  recommends  the  use  of  as  coarse  a  powder 
as  will  be  completely  attacked  by  the  acid.  These  conclusions 
have  been  confirmed,  in  the  main,  by  Hillebrand,4  who  attributes 
the  oxidation  to  strong  local  heating  at  the  moment  of  fracture  of 
the  grains  under  the  pestle.  The  oxidation  can  be  diminished  or 

1  Descriptions  will  be  found  in  the  works  cited  above. 

2  J.  H.  Pratt,  Am.  Jour.  Sci.,  48,  p.  149,  1894. 

3  R.  Mauzelius,  Sver.  Geol.  Unders.,  Aarbog  I,  No.  3,  1907. 

4  Hillebrand,  Jour.  Am.  Chem.  Soc.,  30,  p.  1120,   1908  and  Bull.  422,  p. 
154;  Mellor,  p.  124;  Treadwell,  2,  p.  837;  Sosman  and  Hostetter,  Trans.  Am. 
Inst.  Min.  Eng.,  p.  919,  1917. 


184  METHODS 

even  completely  avoided  by  grinding  under  inert  liquids,  a3  water, 
carbon  tetrachloride,  or  alcohol,  the  last  being  the  most  suitable. 
It  must  be  observed,  as  regards  Hillebrand's  investigations,  that 
the  times  of  grinding  were  very  long,  at  least  two  hours  or  more  in 
most  cases,  a  mechanical  grinder  being  used  and  the  powder  being 
reduced  to  an  extreme  fineness.  When  the  grinding  was  half  an 
hour  or  less  the  oxidation  seems  in  general  to  have  been  neg- 
ligible. 

It  seems  to  follow  from  these  investigations  that  to  obtain 
correct  results  for  FeO,  the  rock  powder  must  be  as  coarse  as  is 
consistent  with  proper  solution  in  the  acids  employed,  and  that 
long-continued  grinding  is  to  be  avoided  in  preparing  the  sample. 
In  the  method  adopted  by  me  for  preparing  the  sample  (p.  68), 
the  time  spent  in  crushing  is  very  short  and  the  grinding  of  the'final 
small  residue  lasts  only  five  minutes  or  less,  so  that  the  oxidation 
of  the  resulting  powder  will  be  negligible.  I  have  also  found 
that  such  a  powder  is  easily  and  completely  attacked  and  dis- 
solved by  the  Pratt  method  in  the  case  of  all  rocks  tested,  ranging 
from  rhyolites  and  granites  to  basalts.  The  main  portion  of  the 
sample  may,  therefore,  serve  for  the  ferrous  oxide  determination 
without  further  preparation  or  grinding.  For  ordinary  rock 
analysis  the  grinding  of  the  powder  under  alcohol  is  not  to  be 
recommended,  as  it  involves  some  sources  of  error  and  loss  of  time 
through  drying. 

On  the  whole,  however,  I  must  advocate  the  special  grinding 
of  the  powder  for  the  ferrous  oxide  determination  because  of  the 
shorter  time  needed  for  complete  decomposition.  Indeed  I  still 
use  it  for  most  rocks,  especially  those  that  contain  considerable 
ferromagnesian  minerals,  pyroxenes,  amphiboles,  and  biotites, 
which  are  attacked  with  difficulty  by  the  acids  used.  As  an  illus- 
tration of  the  data  obtained  in  practice  which  have  led  me  to  the 
retention  of  this  procedure,  I  may  cite  two  determinations  of 
FeO  recently  made  on  the  same  sample  of  an  aegirite  from  Laven. 
The  first  was  made  without,  and  the  second  with,  fine  grinding; 
the  results  were,  respectively,  6.85  and  6.88  per  cent.1  With  such 
rocks  as  granites,  rhyolites,  or  trachytes,  that  contain  but  little 
iron  and  are  readily  decomposed,  the  fine  grinding  may  well  be 
omitted. 

1  These  were  made  without  boric  acid. 


FERROUS  OXIDE  185 

There  is  always  present  the  possibility  of  partial  oxidation, 
both  by  atmospheric  oxygen  and  by  the  sulphuric  acid,  during 
the  decomposition,  and  this  may  be  very  serious.  It  can  be 
guarded  against  and  eliminated  by  careful  manipulation,  but 
uniform  success  as  regards  this  point  is  only  to  be  attained  by 
practice  and  experience. 

If  sulphides,  vanadium  as  ¥263,  or  organic  matter  are  present 
they  will  reduce  the  permanganate  and  render  the  results  too  high. 
Organic  matter  is  not  to  be  generally  expected  in  igneous  rocks, 
and  in  the  great  majority  of  them  sulphides  and  vanadium  are 
only  present  (if  at  all)  in  amounts  so  small  as  to  make  their  influ- 
ence quite  negligible.  This  point  is  discussed  by  Hillebrand,1 
who  may  be  consulted  for  details.  If  any  considerable  amount,  of 
sulphides  is  present  there  would  seem  to  be  no  method  now 
known  to  obtain  correct  figures  for  ferrous  oxide. 

The  presence  of  hydrofluoric  acid  in  the  solution  is  a  very  im- 
portant factor.  While  solutions  of  ferrous  oxide  in  dilute  sul- 
phuric acid  are  fairly  stable,  they  are  much  less  so  if  hydrofluoric 
acid  be  added.  Furthermore  in  the  presence  of  this  acid  the  end 
point  is  transitory,  and  often  several  cubic  centimeters  of  per- 
manganate can  be  added  without  obtaining  a  permanent  colora- 
tion; so  that  it  is  often  difficult  to  finish  the  titration  with  cer- 
tainty or  accuracy. 

Manganous  sulphate  is  oxidizable  by  permanganate,  so  that 
if  the  rock  contains  much  iron,  the  very  considerable  amount  of 
manganous  salt  formed  by  the  reaction  of  the  permanganate  on  the 
ferrous  oxide  also  renders  the  end-point  fleeting  and  the  titration 
somewhat  uncertain. 

Thus  the  presence  of  both  hydrofluoric  acid  and  manganous 
sulphate  are  deleterious,  and  as  they  are  both  necessarily  present 
in  the  analysis  of  rocks,  it  is  obvious  that  the  determination  of 
ferrous  oxide  in  them  must  often  be  looked  on  with  some  suspicion 
as  to  its  accuracy. 

Several  suggestions  have  been  made  to  counteract  these  dele- 
terious effects,  that  of  hydrofluoric  acid  especially.  Thus,  Gage  2 
proposed  the  addition  of  calcium  phosphate,  with  the  view  of 
removing  the  fluorine  from  the  solution  by  precipitation  as  cal- 

1  Hillebrand,  Bull.  422,  pp.  165-166. 

2  R.  B.  Gage,  Jour.  Am.  Chem.  Soc.,  31,  p.  381,  1909. 


186  METHODS 

cium  fluoride.  Fromme  l  converts  the  hydrofluoric  into  fluosilicic 
acid  by  the  addition  of  silicic  acid,  while  Dittrich  2  uses  potassium 
sulphate  and  silicic  acid. 

The  matter  has  recently  been  investigated  thoroughly  by 
Barnebey,3  who  studied  the  effect  of  a  number  of  additions.  He 
finds  that  those  proposed,  as  well  as  some  others,  are  not  entirely 
effective  or  satisfactory,  but  that  the  presence  of  boric  acid  in 
excess  renders  the  solution  stable  in  presence  of  air,  and  gives  a 
definite  and  lasting  end-point.  Indeed,  the  addition  of  boric 
acid  would  seem  to  have  solved  the  problem  of  the  exact  titration  of 
ferrous  oxide  in  silicate  rocks. 

It  must  not  be  thought,  however,  that  it  is  essential,  as  exact 
and  concordant  determinations  can  be  made  by  an  experienced 
analyst  without  its  use.  Thus  Hillebrand4  says:  "It  is  possible 
to  titrate  ferrous  iron  in  presence  of  sulphuric  and  as  much  as  5  to 
7  c.c.  of  40  per  cent  hydrofluoric  acid  in  a  total  volume  of  200  to 
400  c.c.  almost  if  not  quite  as  exactly  as  in  sulphuric  acid  alone, 
provided  that  the  iron  solution  is  diluted  with  air-free  water  and 
the  titration  is  made  immediately  after  adding  the  hydrofluoric 
acid  and  with  all  possible  dispatch."  For  the  beginner,  however, 
and  even  for  the  experienced  analyst,  it  will  be  well  to  follow  Barne- 
bey's  recommendation  and  titrate  in  the  presence  of  boric  acid. 

There  is  a  decided  tendency  to  minus  errors  in  the  determina- 
tion of  ferrous  oxide. 

Simple  Method. — There  are  several  modifications  of  the 
method  of  decomposition  by  a  mixture  of  hydrofluoric  and  sul- 
phuric acids,  which  differ  in  comparative  simplicity  and,  to  some 
extent  also,  in  accuracy.  The  simplest,  and  the  one  which  I  have 
found  to  be  sufficiently  accurate  for  most  purposes  and  by  far  the 
most  rapid,  will  be  described  first.  This  is  essentially  the  method 
first  tested  by  Pratt,  and  is  simpler  than  that  now  adopted  by  the 
U.  S.  Geological  Survey.  I  have  found  it  to  give  accurate  and 
concordant  results. 

About  half  a  gram  of  the  rock  powder  5  is  weighed  into  a  35-c.c. 

1  J.  Fromme,  Tscher.  Min.  Pet.  Mitth.,  28,  p.  329,  1909. 

2  M.  Dittrich,  Cf.  Chem.  Abstr.,  6,  p.  846,  1911. 

3  O.  L.  Barnebey,  Jour.  Am.  Chem.  Soc.,  37,  p.  1481,  1915. 

4  Hillebrand,  Bull.  522,  p.  161. 

5  This  may  or  may  not  be  specially  ground.     See  page  183. 


FERROUS  OXIDE  187 

platinum  crucible,1  the  cover  of  which  must  fit  closely.  It  is 
moistened  with  a  little  water,  this  being  blown  in  gently  against 
the  side  of  the  crucible,  the  tip  of  the  wash-bottle  being  inserted 
beneath  the  slightly  raised  cover.  In  this  way  the  mass  may  be 
wet  without  blowing  out  any  of  the  rock  powder.  When  thor- 
oughly wet  and  pasty,  a  few  small  coils  of  platinum  wire  are  dropped 
in,  to  prevent  bumping.  If  carbonates  are  present  in  the  rock,  a 
few  drops  of  very  dilute  sulphuric  acid  is  poured  in  and  the  cover 
quickly  put  on,  until  effervescence  ceases. 

In  the  meantime  the  nearly  full  hot  water  wash-bottle  is  heated 
until  the  water  boils  vigorously  so  as  to  drive  out  all  dissolved  air; 
after  which  it  is  cooled  under  the  tap. 

In  another  platinum  crucible  or  the  small  basin,  about  10  c.c. 
of  warm  dilute  sulphuric  acid  is  made  by  slowly  pouring  5  c.c.  of 
the  concentrated  acid  into  5  c.c.  or  so  of  the  cold,  boiled  water. 
Into  this  is  poured  about  5  c.c.  of  hydrofluoric  acid,  an  operation 
that  must  be  done  with  caution  and  in  small  portions  at  a  time. 
Hydrofluoric  acid  causes  painful  and  lasting  sores  and  too  great 
caution  cannot  be  exercised  in  handling  it. 

The  crucible  containing  the  wet  rock  powder  is  placed  in  a  tri- 
angle resting  on  a  ring  of  a  retort  stand  in  the  hood.  It  must  not 
be  pressed  into  place,  but  laid  in  loosely  so  that  it  will  be  easily 
removable.  The  ring  should  be  at  a  height  so  as  to  bring  it  about 
10  cm.  above  a  small  Bunsen  burner  flame.  The  Bunsen  burner  is 
lighted,  its  flame  adjusted  to  a  height  of  about  2  cm.  high,  and  is 
placed  near  by,  not  beneath  the  crucible. 

The  crucible  is  now  uncovered  with  the  left  hand,  enough  of 
the  warm  mixture  of  sulphuric  and  hydrofluoric  acids  is  gently 
but  quickly  poured  in  so  as  to  fill  the  crucible  about  half -full,  and 
the  cover  is  quickly  put  in  place,  but  not  pressed  down  tight. 

With  the  Bunsen  burner  waved  somewhat  below  the  crucible, 
the  liquid  is  heated  until  it  just  begins  to  simmer,  but  not  boil, 
when  the  flame  is  slowly  lowered  and  the  burner  put  in  place 
below  the  crucible.  The  points  to  be  attended  to  are  that  the 
hot  liquid  should  begin  to  steam  and  so  replace  the  air  in  the 
crucible  as  soon  as  possible,  and  should  be  kept  simmering  con- 
tinuously, but  without  any  bumping  or  foaming  so  as  to  cause  loss 
by  spattering  or  boiling  over. 

1  A  large  crucible  is  necessary  to  prevent  boiling  over. 


188  METHODS 

This  may  be  attained  by  properly  adjusting  the  size  of  flame 
and  the  height  of  the  crucible  above  it.  These  will  vary,  of  course, 
with  different  conditions,  but  I  have  found  that  with  a  flame 
about  2  cm.  high  the  40-c.c.  crucible  used  for  this  operation  should 
be  about  15  cm.  above  its  tip.  Some  practice  is  needed  but  the 
right  conditions  will  soon  be  learned.  At  first  the  crucible  must 
be  watched,  and  the  operation  proceeds  properly  when  the  liquid 
is  heard  to  be  simmering  gently  and  a  little  white  vapor  issues 
from  around  the  cover.  The  liquid  must  never  "  bump." 

When  all  is  proceeding  well,  or  before  the  acid  is  added,  a 
600-c.c.  beaker  1  is  slightly  more  than  half  filled  with  the  cold 
boiled  water,  about  5  or  10  c.c.  of  dilute  (1  :  1)  sulphuric  acid  are 
poured  in,  and  (following  Barnebey)  2  or  3  grams  of  solid  boric 
acid  are  added.  This  beaker  is  placed  near  the  heated  crucible. 

The  simmering  is  continued  for  five  to  eight  minutes,  accord- 
ing as  the  rock  is  largely  feldspathic  or  is  rich  in  ferromagnesian 
minerals.  For  the  great  majority  of  rocks  I  find  that  six  minutes 
is  ample  for  complete  decomposition,  and  yet  not  long  enough  to 
cause  sensible  oxidation  of  the  ferrous  oxide  by  the  hot  sulphuric 
acid.  The  time  is  best  noted  by  one's  watch  laid  on  the  bench 
near  by. 

When  the  allotted  time  is  up  the  crucible  is  firmly  grasped  in  a 
pair  of  Blair's  tongs,  rather  above  the  center,  and  is  plunged 
quickly  but  without  any  splashing  into  the  water  in  the  600-c.c. 
beaker  standing  at  hand  and  the  tongs  are  immediately  with- 
drawn. The  tongs  are  best  provided  with  platinum  shoes,  but 
those  of  German  silver  or  brass  will  answer.  Tongs  of  the  ordinary 
form  are  not  so  well  adapted  for  this,  as  they  cannot  grasp  the 
covered  crucible  firmly  without  danger  of  slipping  or  letting  the 
crucible  tilt.2 

The  contents  of  the  beaker  are  to  be  titrated  with  standard 
permanganate  solution  immediately  after  the  crucible  has  been 

1  For  very  exact  work  the  lower  half  of  a  large  ceresine  bottle  may  be  used 
in  place  of  a  glass  beaker. 

2  In  default  of  the  proper  tongs  the  crucible  may  be  lifted  with  the  handle 
portions  of  two  ordinary  test-tube  holders  (the  smaller  piece  being  removed), 
with  the  ends  slightly  hollowed  so  as  to  fit  the  crucible.     These  are  held  ver- 
tically, one  in  each  hand,  and  the  crucible  is  grasped  firmly  on  either  side 
rather  near  the  top.     As  suggested  by  Mellor,  a  loop  of  platinum  wire  may  be 
previously  fixed  around  the  crucible  extending  (say  about  10  cm.)  above  it. 


FERROUS  OXIDE  189 

dropped  in;  the  burette  being  already  filled,  weighed,  and  in 
place  for  this  purpose.  During  the  titration  the  liquid  is  gently 
stirred  with  a  glass  rod,  and  the  crucible  and  cover  are  moved 
about  so  that  the  permanganate  may  reach  all  parts  of  it  and  the 
contents  of  the  crucible  be  thoroughly  emptied  and  mixed  with  the 
liquid  in  the  beaker. 

The  titration  is  carried  out  to  the  first  pink  blush  that  persists 
for  a  few  seconds  in  spite  of  stirring.  After  titration  the  contents 
of  the  beaker  should  be  examined  to  see  if  decomposition  has  been 
complete,  which  will  be  shown  by  the  absence  of  hard,  gritty,  or 
dark  particles.  If  such  are  present  the  operation  should  be  carried 
out  again  on  another  weighed  portion  of  powder,  this  being 
ground  specially  fine,  if  it  has  not  been  before,  or  the  heating  con- 
tinued for  a  longer  time. 

With  rocks  that  contain  much  lime  or  magnesia  the  liquid  will 
be  somewhat  turbid.  This  is  not  an  indication  that  the  decom- 
position is  incomplete,  but  is  due  to  the  formation  of  calcium 
sulphate  and  fluorides  of  calcium  and  magnesium. 

The  burette  is  now  weighed.  The  loss  in  weight  (or  the  num- 
ber of  cubic  centimeters  if  an  ordinary  burette  is  used),  is  mul- 
tiplied by  the  FeO  value  of  1  gram  (or  c.c.)  of  the  standard  per- 
manganate solution;  the  product,  divided  by  the  weight  of  rock 
powder  taken,  is  the  percentage  of  FeO.  The  loss  in  weight 
(or  number  of  cubic  centimeters)  is  also  to  be  multiplied  by  the 
Fe20s  value  of  1  gram  (or  c.c.),  to  arrive  at  the  percentage  of 
Fe2O3  that  corresponds  to  that  of  FeO.  The  weight  of  rock 
powder  taken  for  the  "  main  "  portion,  in  which  the  total  iron 
oxides  has  been  determined,  is  multiplied  by  this,  and  the  product 
subtracted  from  the  weight  of  the  total  iron  oxides  (pp.  167,  244). 
The  remainder  is  the  weight  of  the  Fe2Oa  of  the  rock. 

It  may  sometimes  happen,  especially  with  rocks  rich  in  nephe- 
lite  or  minerals  which  gelatinize  with  acids,  that  the  powder  cakes 
at  the  bottom  of  the  crucible,  preventing  complete  decomposition. 
This  is  usually  due  to  the  powder  not  having  been  thoroughly 
stirred  up  with  enough  water  before  the  addition  of  the  mixed 
acids.  In  such  a  case  the  best  remedy  is  to  repeat  the  whole 
operation  until  successful,  or  the  rock  powder  may  be  intimately 
mixed  with  powdered  quartz.1 

1  Suggested  by  Dittrich;  see  Mellor,  p.  462. 


190  METHODS 

After  titration  the  liquid  is  to  be  poured  into  the  sink  and  the 
beaker  well  washed,  so  as  to  prevent  its  corrosion  by  the  hydro- 
fluoric acid. 

Pratt's  Method. — The  simple  method  described  above  was 
modified  by  Pratt,1  by  allowing  a  current  of  carbon  dioxide  to 
flow  into  the  crucible  during  the  boiling  by  a  platinum  tube 
passing  through  a  hole  in  the  cover.  As  modified  by  Hillebrand  2 
the  method  now  employed  in  the  Survey  Laboratory  is  as  follows : 
The  rock  powder  in  the  crucible  is  mixed  with  10  c.c.  of  dilute 
sulphuric  acid,  the  crucible  is  placed  in  a  triangle  over  the  burner, 
and  the  air  is  replaced  by  carbon  dioxide,  which  enters  beneath 
the  lid  slightly  raised  on  one  side.  Before  the  liquid  boils  the  gas 
current  is  stopped  and  the  well-fitting  lid  is  lowered;  5  to  7  c.c. 
of  strong  hydrofluoric  acid  are  then  quickly  poured  in  through 
an  opening  formed  by  drawing  the  lid  a  trifle  to  one  side,  the  lid 
is  replaced,  and  the  boiling  continued  for  the  requisite  time.  '  The 
rest  of  the  operation  is  as  above.  This  method  has  certain  appa- 
rent advantages  over  the  first,  but  I  am  inclined  to  think  that 
there  is  little  to  choose  between  them,  so  far  as  accuracy  is  con- 
cerned. 

Mellor  (p.  464)  heats  the  crucible  beneath  a  15-inch  funnel 
(paraffine-coated),  through  which  a  stream  of  carbon  dioxide 
flows.  The  crucible  is  supported  in  a  hole  in  a  square  of  asbestos 
board,  on  which  the  inverted  funnel  rests.  This  is  a  simple,  and 
should  be  an  effective,  arrangement. 

If  an  appreciable  amount  of  sulphur  as  sulphides  exists  in  the 
rock,  regard  must  be  had  to  the  iron  in  combination  with  it. 
If  pyrrhotite  is  the  only  sulphide  present,  this  will  be  decomposed 
by  the  mixture  of  acids  in  the  determination  of  ferrous  oxide,  and 
the  iron  will  appear  as  FeO.  The  sulphur  may  be  either  stated 
as  S  in  the  analysis,  or  the  amount  of  iron  necessary,  for  the  mole- 
cule FeySs  of  pyrrhotite  calculated  and  deducted  from  the  amount 
of  FeO,  and  the  percentage  of  pyrrhotite  given.  The  former  pro- 
cedure is  rather  the  better.  If  the  only  sulphide  is  pyrite,  this 
will  not  be  attacked  in  the  determination  of  FeO,  but  the  iron  in 
this  mineral  will  appear  as  Fe20s.  This  may  be  accorded  treat- 
ment similar  to  the  iron  in  pyrrhotite.  If  both  sulphides  are 

1  J.  H.  Pratt,  Am.  Jour.  Sci.  (3)  48,  p.  149,  1894. 

2  Hillebrand,  p.  167. 


POTASH  AND  SODA  191 

present,  it  will  be  impossible  to  estimate  the  real  correction 
unless  the  relative  amounts  of  the  two  minerals  are  known. 
Fortunately  this  is  seldom  needed,  and  in  general  the  amount  of 
sulphur  is  so  small  that  corrections  for  it  are  not  often  necessary. 

13.  POTASH  AND  SOD  A1 

Two  prominent  methods  are  available  for  the  determination  of 
the  soda  and  potash.  They  differ  in  the  means  employed  for  the 
decomposition  of  the  rock  and  for  the  elimination  of  all  the  other 
constituents,  the  object  of  both  being  to  obtain  the  alkali  metals 
alone  in  solution  as  chlorides,  and  the  final  separation  of  these. 

In  the  older  method  the  rock  powder  is  decomposed  by  a 
mixture  of  sulphuric  and  hydrofluoric  acids,  or  by  fusion  with 
bismuth,  lead,  or  boric  oxides,  digestion  with  the  acid  mixture 
being  that  most  used.  The  solution  obtained  from  this  is  treated 
successively  with  ammonia  and  with  ammonium  oxalate  to  remove 
silica,  alumina,  iron,  titanium,  phosphorus  and  lime.  The  mag- 
nesia is  separated  by  one  of  several  reagents  (most  often  by  the 
use  of  HgO),  the  sulphuric  acid  is  removed  by  lead  acetate  or 
barium  chloride,  and  the  alkalies  are  determined  in  the  filtrate  as  in 
the  method  described  below.  Or  barium  hydrate  may  be  used 
to  separate  the  other  constituents  from  the  alkalies  (Classen). 
It  is  clear  that  any  of  these  processes  is  long  and  complex,  and 
that,  not  only  do  they  suffer  from  the  length  of  time  needed,  but 
that  there  is  danger  of  loss  of  alkalies  during  the  blasting  neces- 
sary with  some  of  the  fluxes.  Still  more,  the  final  solution  is 
liable  to  be  contaminated  by  alkalies  derived  from  the  many 
reagents  used  and  taken  up  from  the  glass  vessels.  This  method 
should  never  be  used;  in  the  United  States  it  has  been  completely 
superseded  by  the  Smith  method. 

The  second  method  is  that  of  J.  Lawrence  Smith.2  It  con- 
sists in  decomposing  the  rock  by  heating  with  calcium  carbonate 
and  ammonium  chloride,  leaching  with  water  from  the  insoluble 
silicate  and  aluminate  of  calcium,  and  carbonates  of  iron,  cal- 

1  For  the  Smith  method  see:  J.  L.  Smith,  Am.  Jour  Sci.,  1,  p.  269,  1871; 
Classen,  2,  pp.  613-615;  Fresenius,  1,  pp.  519-520;  Hillebrand,  pp.  171-174; 
Mellor,  pp.  222-226,  231-237;  Treadwell,  2,  pp.  496-499. 

2  J.  L.  Smith,  Am.  Jour.  Sci.,  1,  p.  269,  1871. 


192  METHODS 

cium  and  magnesium,  precipitation  of  the  rest  of  the  lime  by 
ammonium  carbonate,  expulsion  of  ammonium  salts  by  heating 
the  evaporated  filtrate,  and  final  separation  of  the  alkalies  by 
chloroplatinic  acid. 

The  advantages  of  this  method  are:  its  convenience  and  expe- 
dition, the  manipulations  being  few,  and  a  day  and  a  half  or  two 
days  being  ample  for  the  complete  determination;  the  separation 
of  magnesia  at  the  start,  which  is  a  troublesome  constituent  to 
separate  from  the  alkalies  by  the  other  methods;  the  small  danger 
of  introduction  of  alkalies  from  reagents  or  glass  vessels,  and, 
finally,  its  great  accuracy,  which  is  fully  equal,  if  not  superior, 
to  that  of  the  older  methods.1  The  only  real  objection  which 
can  be  urged  against  this  method,  as  compared  with  the  other, 
is  the  difficulty  of  obtaining  a  calcium  carbonate  entirely  free 
from  alkalies.  The  amount  of  these,  however,  is  easily  ren- 
dered extremely  small  by  prolonged  washing,  and  it  is  a  con- 
stant error,  the  correction  for  which  can  be  safely  applied  when 
once  determined  for  the  stock  of  calcium  carbonate.  Even  if 
this  is  not  done,  however,  it  is  certain  that  the  errors  involved 
will  be  less  than  those  incident  to  the  other  methods  if  care  be 
employed  in  the  preparation  of  the  calcium  carbonate. 

This  method  is  practically  the  only  one  which  has  been  used 
by  the  chemists  of  the  U.  S.  Geological  Survey,  of  the  extreme 
accuracy  and  almost  uniquely  high  character  of  whose  analyses 
there  can  be  no  question.  It  is  likewise  the  method  which  I  have 
adopted  exclusively,  and  which  is  almost  universally  employed 
in  this  country.  In  Europe,  on  the  other  hand,  it  seems  to  be 
less  well  known,  or  at  least  little  used,  and  its  undoubted  merit  and 
superiority  over  the  other  are  not  so  generally  recognized.  Only 
the  J.  Lawrence  Smith  method  will  be  described  here. 

Errors. — An  inherent  error  is  that  the  calcium  carbonate  used 
is  seldom  to  be  obtained  entirely  free  from  alkalies,  very  small 
amounts  of  sodium,  and  much  less  potassium  carbonate  being 
present.  These  can  be  reduced  to  negligible  amount  by  thor- 
ough washing  of  the  properly  precipitated  carbonate  with  hot 
water,  but  even  if  they  are  present  they  are  of  slight  importance, 
as  their  amount  can  be  determined  once  for  all  in  the  stock  of 

1CL  J.  L.  Smith,  loc.  cit.;  Hillebrand,  Bull.  422,  p.  171;  M.  Dittrich, 
Neues  Jahrbuch,  2,  p.  81,  1903. 


POTASH  AND  SODA  193 

calcium  carbonate  and  an  appropriate  correction  is  easily  and 
safely  applied.  If  the  calcium  carbonate  is  properly  prepared 
and  well  washed,  the  error  involved  by  neglect  to  apply  this 
correction  will  seldom  be  serious. 

Another  source  of  error  is  the  retention  of  alkalies  in  the  fused 
cake.  This  has  been  investigated  by  Hunter 1  and  Connor.2 
Their  results  show  that  there  is  apt  to  be  such  a  loss,  as  was 
already  pointed  out  by  Smith.  As  their  results  differ  in  magni- 
tude, they  indicate  that  the  loss  is  dependent  on  the  manipulation. 
If  the  process  is  properly  carried  out  the  loss  should  be  small, 
but  in  highly  accurate  work  the  washed  cake  should  be  again 
treated  with  some  ammonium  chloride  and  a  little  additional  cal- 
cium carbonate. 

Overheating  of  the  crucible  during  the  sintering  with  calcium 
carbonate,  overheating  or  too  rapid  heating  of  the  basin  during  the 
driving  off  of  the  ammonium  chloride,  or  of  the  crucible  in  drying 
the  mixed  chlorides,  are  all  liable  to  lead  to  vaporization  of  the 
sodium  and  potassium  chlorides.  This  can  be  entirely  prevented 
by  careful  attention  to  the  conditions  of  heating  as  described  on 
later  pages.  Loss  by  decrepitation  of  the  chlorides  is  to  be 
guarded  against. 

The  weight  of  the  carbon  sometimes  found  in  the  final  mixed 
chlorides  is  so  small  as  to  be  negligible,  as  shown  by  Smith  and 
confirmed  by  my  own  experience. 

Smith  Method. — For  the  determination  of  the  alkalies  a 
specially  ground  portion  of  rock  powder  is  to  be  used.  Although 
Smith  states  that  this  is  not  absolutely  necessary  in  all  cases,  yet 
it  is  certainly  advisable,  as  complete  decomposition  can  be  secured 
at  a  lower  temperature  and  with  more  certainty  than  if  the  powder 
be  coarse. 

A  little  more  than  1  gram  of  the  rock  powder  is  ground  down 
by  hand  in  a  small  agate  mortar,  the  grinding  being  continued 
until  a  small  pinch  causes  no  gritty  feeling  when  rubbed  on  a 
tender  part  of  the  skin,  such  as  the  web  connecting  the  thumb  and 
the  index  finger  or  the  inside  of  the  wrist.  It  is  then  placed  in  a 
small  special  specimen  tube,  corked  and  numbered.  No  appre- 
ciable error  due  to  partial  oxidation  of  the  FeO  is  to  be  feared 

1  See  Mellor,  p.  225. 

2  M.  F.  Connor,  XII  Cong.  Geol.  Int.,  C.  R.,  p.  888,  1914. 


194  METHODS 

by  this  procedure,  and  it  does  not  involve  the  great  chance  of  loss 
of  the  fine  powder  incident  to  the  method  of  first  weighing  out  the 
coarse  powder  and  subsequent  grinding.  About  one-half  of  this 
powder  will  serve  for  the  ferrous  oxide  determination,  if  such 
specially  ground  powder  is  to  be  used  for  this.  About  \  gram  is 
used  for  the  alkali  determination. 

As  the  rock  powder  has  to  be  more  than  ordinarily  well  mixed 
with  the  flux,  the  preparation  of  the  portion  used  demands  a  pro- 
cedure different  from  that  used  for  the  portions  for  other  constit- 
uents. 

If  the  analyst  has  had  some  experience,  the  portion  may  be 
weighed  out  by  the  method  by  subtraction  described  on  p.  130. 
Special  care  must  be  taken  to  prevent  loss  of  any  of  the  fine 
powder. 

The  small  tube  containing  the  specially  ground  powder  is  wiped 
perfectly  dry,  uncorked,  placed  on  the  pan  on  the  small  frame 
intended  for  this  purpose  and  weighed. 

After  weighing,  and  noting  the  weight  as  Tube+Subst.,  a 
half-gram  weight  is  removed  from  the  right-hand  pan,  and  about 
half  a  gram  of  powder  is  carefully  poured  out  into  the  platinum 
basin.  This  must  be  done  with  care  to  avoid  any  loss  of  powder, 
and  when  a  sufficient  quantity  has  been  poured  out,  the  tube  is  to 
be  gently  tilted  up  and  lightly  tapped  to  bring  the  powder  down 
toward  the  bottom,  the  mouth  being  held  over  the  basin.  Not 
more  than  0.6  gram  need  be  taken,  but  not  less  than  0.45  gram. 
Half  a  gram  is  quite  sufficient  to  yield  results  fully  as  accurate  as 
1  gram,  and  the  consequent  saving  of  platinum  as  well  as  shortening 
of  the  time  needed  for  the  determination  are  rather  important 
considerations.  The  tube  (still  uncorked)  is  then  weighed  again, 
and  the  difference  between  this  weight,  recorded  as  Tube  — Subst., 
and  the  former  gives  the  weight  of  powder  taken. 

Just  as  in  the  weighing  out  of  the  powder  for  the  alkali  car- 
bonate fusion,  it  may  be  necessary  to  shake  out  and  weigh  addi- 
tional small  portions  several  times.  The  endeavor  should  be 
made  to  get  the  final  weight  only  slightly  above,  and  as  near  to 
0.5  gram  as  possible  without  undue  loss  of  time.  A  little  prac- 
tice enables  one  to  do  this  very  quickly. 

Under  no  circumstances  is  any  powder  to  be  taken  up  with  the 
spatula  and  replaced  in  the  tube,  so  as  to  bring  the  weight  down 


POTASH  AND  SODA  195 

to  neaily  J  gram  in  case  too  much  has  been  poured  out.  If  this 
last  has  been  done,  either  the  determination  is  to  be  made  on  the 
somewhat  large  amount,  which  will  be  not  at  all  objectionable,  or 
the  powder  is  poured  back  into  the  tube,  the  basin  wiped  out  care- 
fully, and  the  weighing  done  again,  more  care  being  taken  this 
time  in  pouring  out  the  powder. 

One  must  be  especially  careful  to  avoid  any  loss  by  wafting 
away  of  the  fine  powder.  The  breath  should  not  be  directed 
toward  the  basin  while  the  powder  is  being  poured  out,  and  the 
basin  is  to  be  covered  with  a  watch-glass  immediately  after. 

The  balance-pans  are  then  cleared  of  frame  and  weights,  the 
pair  of  balanced  watch-glasses  are  substituted,  and  an  amount  of 
dry  ammonium  chloride  equal  to  that  of  the  rock  powder  taken 
is  weighed  out.  It  is  not  necessary  that  this  weight  be  exact, 
and  it  may  be  a  decigram  or  some  more,  but  should  not  be  less. 
This  is  poured  into  the  basin,  above  the  heap  of  rock  powder,  but 
very  slowly  so  as  not  to  disturb  and  lose  any  of  this. 

The  beginner  would  best  use  the  other  method  and  weigh  out 
about  |  gram  into  the  30-cc.  platinum  crucible  that  is  later  to  be 
used  for  the  fusion.  This  weighing  is  by  the  method  by  addition, 
described  on  p.  129.  About  J  gram  of  ammonium  chloride  is 
then  roughly  weighed  out  into  the  crucible  above  the  rock  powder, 
with  precautions  to  avoid  loss  of  the  latter.  If  too  much  of  the 
chloride  is  poured  in,  the  excess  should  not  be  removed,  for  fear 
that  some  of  the  rock  powder  might  be  taken  out  at  the  same 
time.  A  small  excess  of  the  chloride  does  no  harm. 

The  rock  powder  and  the  ammonium  chloride  are  well  mixed 
in  the  crucible  with  the  small  end  of  the  platinum  spatula,  the 
slight  dampness  of  the  ammonium  salt  serving  to  prevent  loss  of 
the  rock  powder.  The  end  of  the  spatula  is  cleaned  off  on  a  little 
ammonium  chloride  (in  the  watch-glass)  which  is  added  to  that 
in  the  crucible.  The  platinum  basin  is  inverted  over  the  crucible 
and  the  whole  turned  over,  so  that  the  powder  falls  into  the  basin 
without  loss.  The  basin  is  covered  and  the  rest  of  the  procedure 
is  the  same  whatever  method  of  weighing  has  been  used. 

The  use  of  the  platinum  basin  is  preferred  as  a  mixing-vessel 
to  that  of  a  large  agate  mortar,  as  recommended  by  Hillebrand, 
because,  while  the  mixture  may  be  made  just  as  thorough,  there 
is  less  liability  to  loss  owing  to  the  high  sides  of  the  basin,  and 


196  METHODS 

because  the  mixed  powders  are  transferred  far  more  easily  and 
safely  to  the  crucible  from  the  basin  than  from  the  mortar.  I 
also  prefer  to  have  the  powder  ground  specially  fine  before  weigh- 
ing, instead  of  after,  as  recommended  by  Hillebrand,  as  the  latter 
is  almost  certain  to  lead  to  loss. 

Whichever  be  the  method  of  weighing  used,  the  rock  powder 
and  the  ammonium  chloride  are  thoroughly  mixed,  by  rubbing 
with  the  small  agate  pestle.  This  mixing  must  be  very  thorough 
and  must  be  effected  without  any  loss  of  powder.  Attention 
should  be  paid  to  the  powder  at  the  sides,  and  an  epicycloidal 
motion  is  effective. 

An  amount  of  calcium  carbonate  equal  to  eight  times  the 
weight  of  rock  powder  (about  4  grams)  is  then  weighed  on  one 
of  the  pair  of  watch-glasses.  If  a  correction  for  the  small  amount  of 
alkalies  present  is  to  be  made  this  weighing  should  be  carried  out 
to  centigrams,  but  if  not,  the  weighing  need  be  only  approximate, 
but  should  be  more,  rather  than  less  than  the  required  amount. 
With  basic  rocks  5  grams  should  be  used,  to  prevent  too  great 
fluidity  during  the  fusion.  A  small  amount  of  calcium  carbonate  is 
transferred  by  the  platinum  spatula  to  an  unweighed  30-c.c. 
platinum  crucible,  just  sufficient  to  cover  the  bottom,  and  pressed 
lightly  down  with  the  small  agate  pestle.  This  prevents  any 
adhesion  of  the  fused  cake  to  the  bottom  of  the  crucible. 

After  the  rock  powder  has  been  mixed  with  the  ammonium 
chloride,  the  mixture  is  mixed  with  the  greater  part  of  the  calcium 
carbonate,  also  in  the  platinum  basin.  It  is  best  to  add  the  car- 
bonate in  two  or  three  portions,  and  the  rubbing  with  the  pestle 
must  be  carefully  but  thoroughly  done.  The  object  is  to  have,  so 
far  as  possible,  some  ammonium  chloride  and  calcium  carbonate 
in  contact  with  each  particle  of  rock. 

When  the  mixing  is  considered  complete,  it  is  well  to  con- 
tinue it  for  a  few  minutes  longer.  The  pestle  is  laid  down  with 
its  lower  end  in  the  watch-glass,  and  the  mixture  is  poured  cau- 
liously  through  the  lip  of  the  basin  into  the  crucible.1  This 
transfer  is  aided  by  the  platinum  spatula  in  brushing  down  small 
lumps,  and  by  final  gentle  tapping  of  the  spatula  on  the  inside  of 
the  basin,  so  as  to  cause  the  whole  to  pass  through  the  lip  without 

1  It  is  best  to  place  the  crucible  on  a  6-inch  square  of  clean  paper,  so  that 
any  powder  falling  outside  it  may  be  recovered. 


POTASH  AND  SODA  197 

loss  outside  the  crucible.  The  contents  of  the  crucible  are  then 
smoothed  down  with  the  spatula,  the  remaining  calcium  carbonate 
is  poured  into  the  basin  and  the  latter  rinsed  with  it  by  means  of 
the  pestle,  which  is  also  cleaned  at  the  same  time.  The  spatula 
is  cleaned  by  rubbing  against  the  carbonate  in  the  basin,  and  the 
final  portion  of  this  is  transferred  as  before  into  the  crucible. 

If  the  rock  is  specially  ground  as  fine  as  has  been  described, 
the  decomposition  will  be  complete  at  a  temperature  not  high 
enough  to  vaporize  the  alkali  chlorides.  An  ordinary  crucible 
may,  therefore,  be  employed,  with  a  well-fitting  cover,  instead  of 
the  capped  conical  one  recommended  by  Smith  and  by  Hi  lie- 
brand,  which  permits  a  higher  temperature  for  the  sintering.  The 
latter  is,  of  course,  to  be  preferred,  but  it  is  a  somewhat  expensive, 
and  otherwise  unnecessary,  piece  of  platinum,  so  that  it  is  as 
well  to  know  that  perfectly  satisfactory  results  may  be  obtained 
without  its  use. 

The  crucible  is  covered  and  heated  over  a  low  flame  for  ten 
minutes  or  so,  until  no  more  vapors  of  ammonia  or  ammonium 
chloride  are  given  off.  The  heating  is  then  continued  over  the 
one-third  full  flame  of  a  Bunsen  burner,  only  the  lower  third  of  the 
crucible  being  heated  to  a  not  very  bright  red,  and  the  crucible 
being  kept  well  covered.  This  is  continued  for  three-quarters  of 
an  hour,  when  the  crucible  is  allowed  to  cool. 

When  cold,  the  mass  is  soaked  in  the  crucible  with  just  enough 
water  to  cover  it,  and  the  quicklime  that  has  been  formed  is 
allowed  to  slake,  by  which  the  disintegration  is  rendered  almost 
complete.1  By  the  aid  of  the  platinum  spatula  and  a  little  water 
from  the  wash-bottle  the  contents  of  the  crucible  are  easily  trans- 
ferred to  the  platinum  basin,  any  adhering  portions  being  removed 
by  the  spatula.  A  little  water  is  allowed  to  remain  in  the  crucible 
to  soak  it  out.  The  spatula  is  rinsed  off  into  the  basin,  which 
should  contain  not  more  than  about  50  c.c.  of  water. 

The  partially  disintegrated  mass  is  well  rubbed  up  with  the 
agate  pestle,  the  pestle  rinsed  off  with  a  little  water,  the  water  in 
the  crucible  added,  and  the  contents  of  the  basin  are  brought  to  a 
boil,  which  should  be  continued  gently  for  a  few  minutes.  The 
liquid  is  then  decanted  through 'a  9-cm.  filter  into  a  600-c.c. 

1  If  the  rock  contains  much  iron  the  cake  will  have  been  more  or  less  fused, 
and  so  the  disintegration  will  not  be  as  complete  as  it  is  with  most  rocks. 


198  METHODS 

beaker.1  The  stirring-rod  is  rinsed  off  into  the  basin,  and  the  mass 
once  more  rubbed  up  with  the  pestle  till  there  are  no  more  lumps, 
the  pestle  is  finally  rinsed  and  the  basin  again  heated.  The  liquid 
is  decanted  through  the  filter,  the  powder  once  more  heated  to 
boiling  with  a  little  water,  and  finally  the  contents  of  the  basin 
are  brought  on  the  filter.  The  basin  is  rinsed,  and  the  contents 
of  the  filter  are  wash'ed  with  hot  water,  in  small  portions  at  a  time, 
the  powder  being  well  stirred  up  by  the  first  additions  of  water 
from  the  wash-bottle. 

It  is  impossible  to  ascertain  when  the  washing  is  complete  by 
acidifying  drops  of  the  filtrate  with  nitric  acid  and  testing  with 
silver  nitrate,  as  an  oxy chloride  of  calcium  is  formed  which  dis- 
solves slowly  in  water,  and  will  thus  give  a  reaction  for  chlorine 
long  after  all  alkalies  are  washed  out.  Smith  states  that  com- 
plete washing  is  effected  with  200  c.c.  of  water,  but  it  is  as  well 
to  be  on  the  safe  side  and  to  use  250  to  300  c.c.,  which  will  make 
complete  washing  certain.  This  volume  may  be  conveniently 
marked  on  the  600-c.c.  beaker  used  for  this  operation  by  a  thin 
line  of  paint. 

It  will  be  well  for  the  beginner  to  test  the  thoroughness  of  the 
decomposition  by  dissolving  a  portion  of  the  moist  mass  on  the 
filter  in  hydrochloric  acid.  Solution  will  be  complete  if  the 
fusion  has  been  properly  effected. 

To  the  filtrate  a  little  ammonia  water  is  added  and  the  liquid 
brought  to  a  boil.2  About  1.5  to  2  grams  of  ammonium  carbonate 
previously  dissolved  in  25  c.c.  of  water  3  are  then  added,  and  the 
boiling  is  continued  for  a  minute  or  so.  The  lime  is  thus  com- 
pletely precipitated,  with  the  exception  of  a  trace  which  is  sepa- 
rated later,  and  the  alkalies  are  left  in  solution  as  chlorides,  along 
with  ammonia  chloride. 

The  bulky  precipitate  of  calcium  carbonate  is  allowed  to  settle 

1  One  may  advantageously  use  a  400-c.c.  beaker  of  fused  silica,  so  as  to 
avoid  contamination  by  glass. 

2  Addition  of  ammonia  is  necessary  to  prevent  the  formation  of  soluble 
calcium  bicarbonate.     The  iridescent  scum  on  the  surface  of  the  liquid  is, 
of  course,  due  to  the  action  of  atmospheric  CO2  on  the  calcium  hydroxide 
and  chloride. 

3  The  solution  of  this  should  be  begun  when  the  crucible  is  put  over  the 
flame,  so  as  to  have  it  complete  in  time.     It  cannot  be  hastened  by  heating, 
as  this  decomposes  the  ammonium  carbonate. 


POTASH  AND  SODA  199 

a  little,  and  is  then  filtered  through  a  9-cm.  filter  into  a  capacious 
basin  (500  c.c.).  This  is  preferably  of  platinum,  but  as  such  a 
large  one  would  be  very  expensive,  a  fused  silica  basin  can  be 
used  with  equal  accuracy.  In  default  of  this  a  glazed  porcelain 
basin  will  answer,  with  but  slight  danger  of  contamination  by 
alkalies  taken  up  from,  the  glaze.  A  glass  basin  must  not  be  used 
on  any  account,  as  the  liquid  will  be  seriously  contaminated  with 
alkalies  derived  from  it. 

The  precipitate  is  all  brought  on  the  filter,  the  beaker  rinsed 
and  the  contents  of  the  filter  are  washed  with  hot  water  in  small 
portions  at  a  time,  till  there  is  no  chlorine  reaction.  The  volume  of 
liquid  in  the  basin  should  be  from  300  to  400  c.c.  The  basin  is 
placed  on  the  water-bath.  It  is  best  to  leave  it  covered  with  a 
large  watch-glass  for  ten  minutes,  as  the  excess  of  ammonium 
carbonate  is  decomposed  by  the  heat  and  the  liquid  effervesces. 
When  this  is  finished,  the  cover  is  rinsed  into  the  basin,  and  the 
liquid  is  evaporated  down  to  dryness,  which  should  be  complete, 
as  indicated  by  the  white  color  of  the  salts.  This  may  be  done 
conveniently  by  leaving  on  the  water-bath  over  night. 

The  drying  may  be  hastened  materially  by  adding  about  10  c.c. 
of  alcohol  when  the  contents  of  the  basin  are  almost  dry,  and  heat- 
ing to  dryness  on  the  water-bath.  There  is  thus  also  less  liability 
to  decrepitation. 

If  desired  the  contents  of  the  large  basin  after  evaporation 
down  to  about  50  c.c.  may  be  transferred  to  the  platinum  basin 
used  for  the  mixing  of  the  powder  and  flux,  and  the  evaporation 
to  dryness  carried  out  in  this. 

The  basin,  covered  with  a  dry  watch-glass,  is  then  placed  on  a 
square  of  gauze  over  a  low  flame  and  heated  gently.  This  heating 
must  be  cautious,  and  if  there  is  any  decrepitation,  due  to  incom- 
plete drying,  it  should  be  interrupted  frequently  till  the  decrepita- 
tion subsides,  or  otherwise  particles  of  the  salts  may  be  thrown  up 
and  stick  to  the  cover-glass.  If  the  cover  is  slightly  dewed  with 
moisture  at  first,  it  is  well  to  remove  it  frequently  and  wipe  the 
moisture  off  quickly,  so  as  to  avoid  such  a  mishap.  When  decrep- 
itation has  wholly  ceased  and  white  vapors  of  ammonium  chloride 
begin  to  rise,  the  cover  is  taken  off,  the  wire  gauze  removed,  the 
basin  being  left  on  the  ring  of  the  retort  stand.  The  upper  sides 
are  gently  warmed  by  a  half-full  flame  of  the  Bunsen  burner,  which 


200  METHODS 

is  waved  over  the  surface  until  the  sides  are  free  from  ammonium 
chloride. 

The  salts  at  the  bottom  are  next  subjected  to  the  same  operation 
till  no  more  white  vapors  are  given  off.  Great  care  must  be  taken 
that  the  bottom  of  the  basin  is  not  overheated  so  that  the  salts 
melt  and  lead  to  the  possible  vaporization  of  alkali  chlorides. 
During  this  process  the  clear  white  mass  becomes  dark  and  dirty- 
looking,  from  carbonization  of  the  traces  of  organic  matter  which 
even  very  pure  ammonium  carbonate  usually  contains.  Pro- 
longed gentle  heating  will  cause  this  to  disappear  to  a  large  extent, 
but  as  the  carbon  is  removed  by  filtration  its  disappearance  at  this 
stage  is  not  necessary. 

After  cooling,  a  little  water  is  added,  just  enough  to  dissolve 
the  chlorides.  If  the  rock  contains  sulphides,  and  especially  if 
haiiyne  or  noselite  are  present,  a  drop  of  barium-chloride  solution 
is  added  to  precipitate  the  sulphuric  acid,  which  would  otherwise 
appear  later  as  sodium  sulphate  and  lead  to  slight  error.  A  few 
drops  of  the  solution  of  ammonium  carbonate  are  then  added  to 
precipitate  the  excess  of  barium  and  small  amount  of  lime  which  is 
always  present  in  traces  at  this  stage;  or,  if  no  sulphates  are 
present,  a  drop  of  ammonium  oxalate  solution  is  also  added, 
as  this  precipitates  calcium  more  completely  than  the  carbonate. 
After  rinsing  the  interior,  as  high  as  the  salts  extend,  by  gentle 
rocking  and  tipping  of  the  small  bulk  of  liquid,  so  as  to  ensure 
their  complete  solution,  the  basin  is  placed  on  the  water-bath 
and  evaporated  again  almost  to  dryness. 

Two  or  3  c.c.  of  water  are  then  poured  in  to  dissolve  the  salts, 
and  the  small  amount  of  liquid  is  filtered  through  a  5J-cm.  filter 
placed  in  a  3|-cm.  funnel,  without  suction-tube,  into  a  previously 
ignited  and  weighed  35-c.c.  crucible.  The  greatest  care  must  be 
taken  in  pouring  out  the  first  portion  of  liquid,  as  drops  are  apt 
to  fly  out  of  the  filter  if  they  fall  from  a  height.  The  loss  of  a 
single  one  at  this  stage  would  cause  so  relatively  large  a  loss  of  the 
small  bulk  of  concentrated  solution  as  to  necessitate  beginning 
the  determination  over  again  from  the  start.  This  may  be  pre- 
vented by  using  a  small  stirring-rod,  with  which  a  path  of  liquid  is 
streaked  to  the  lip,  having  the  rod  wet  and  its  end  touching  the 
filter  paper.  The  basin  and  filter  are  washed  at  least  half  a  dozen 
times,  preferably  with  warm  water,  and  using  only  as  little  as 


'  POTASH  AND  SODA  201 

possible,  not  over  3  or  4  c.c.  at  a  time.  When  the  washing  is 
complete,  as  shown  by  a  test  for  chlorine  on  a  single  drop  toward 
the  last,  the  crucible  should  not  be  more  than  three-quarters  full. 

A  drop  of  dilute  HC1  may  be  added  to  the  crucible,  to  decom- 
pose any  alkali  carbonates  possibly  present,  it  is  placed  on  the 
water-bath,  and  the  liquid  is  evaporated  to  complete  dryness. 
Care  must  be  taken  to  ensure  this,  as  small  amounts  of  water 
caught  under  the  crust  resist  evaporation  for  a  considerable 
length  of  time.  It  is  not  advisable,  either  here  or  in  the  evapora- 
tion in  the  basin,  to  use  a  platinum  spatula  or  wire  to  break  up 
the  crust  and  hasten  the  operation,  on  account  of  the  danger  of 
loss  of  substance.  The  contents  of  the  crucible  can  usually  be 
rendered  dry  in  three  hours  or  so,  if  the  water  be  kept  at  a  brisk 
boil,  or  may  be  left  over  night. 

When  dry,  the  crucible  is  placed  on  a  platinum  triangle,  cov- 
ered, and  very  gently  heated  with  a  small  flame,  held  in  the  hand 
and  moved  about  at  some  distance  beneath.  When  the  slight 
decrepitation  ceases  and  vapors  of  ammonium  chloride  rise,  the 
flame  is  gradually  raised  (but  not  as  far  as  the  crucible  bottom), 
till  no  more  vapors  are  given  off,  as  may  be  ascertained  by  lifting 
the  cover  from  time  to  time.  The  cover  is  then  freed  from  ammo- 
nium chloride  by  heating  over  the  flame,  and  the  sides  of  the  cru- 
cible are  similarly  treated.  The  salts  at  the  bottom  are  then 
most  cautiously  heated  with  the  small  flame  till  no  more  vapors 
are  given  off  and  the  salts  just  begin  to  melt  in  places.  When  this 
happens  the  flame  is  to  be  removed  instantly.  The  bottom  of  the 
crucible  should  not  be  heated  above  a  very  faint  red,  scarcely 
visible  in  daylight. 

It  is  to  be  remembered  that  one  has,  on  the  one  hand,  to  ensure 
the  dryness  of  the  salts  and  the  complete  expulsion  of  ammonium 
chloride,  which  would  later  be  precipitated  with  the  potassium 
platinichloride ;  and  on  the  other,  to  avoid  any  vaporization  of 
sodium  or  potassium  chlorides,  which,  however,  need  not  be 
feared  if  the  chlorides  are  not  heated  above  their  melting-points. 
.  If  more  than  a  few  drops  of  ammonium  carbonate  or  oxalate 
has  been  used  to  precipitate  the  traces  of  lime,  the  salts  may  be 
darkened  by  deposited  carbon.  This  will  usually  be  burnt  off 
entirely,  or  nearly  so,  in  the  process  of  driving  off  the  ammonium 
chloride  and  the  incipient  fusion  of  the  alkali  chlorides.  The 


202  METHODS 

slight  amount  of  it  usually  remaining  is  practically  unweighable, 
as  my  experience  has  shown,  and  it  may  therefore  be  neglected. 

The  platinum  crucible  containing  the  salts  is  cooled  in  the 
desiccator,  weighed  quickly,  and  the  weight  recorded  as  Cruc. 
+NaCl+KCl.  Five  or  10  c.c.  of  water  are  poured  in  to  dis- 
solve the  salts,  and  if  the  previous  operations  have  been  properly 
conducted  the  solution  will  be  clear,  or  at  most  only  a  few  flakes 
of  carbonaceous  matter  will  be  present,  which  may  be  neglected 
as  explained  above,  unless  the  extreme  of  accuracy  be  required. 
If,  however,  there  is  an  insoluble  residue  of  calcium  carbonate 
the  contents  of  the  crucible  must  be  again  filtered,  without  addi- 
tion of  ammonium  carbonate  or  oxalate,  through  a  small  filter 
into  another  weighed  crucible,  the  filter  washed,  again  evaporated 
to  dry  ness,  and  the  operation  repeated  as  before.  The  crucible 
and  its  now  perfectly  pure  contents  are  weighed,  and  this  weight 
and  that  of  the  new  crucible  substituted  for  the  former  ones.  It 
will  be  found  that  the  difference  seldom  amounts  to  more  than  a 
few  tenths  of  a  milligram. 

Separation  of  Potash. — We  have  now  to  separate  the  potash 
and  soda  so  as  to  determine  each  of  them.  For  this  there  are  two 
appropriate  reagents. 

The  one  that  is  by  far  the  best  and  most  used  is  chloroplatinic 
acid,  which  converts  the  alkali  chlorides  into  platinichlorides.1 
Potassium  platinichloride  is  practically  insoluble  in  strong  alcohol, 
in  which  the  sodium  salt  dissolves  readily.  This  reagent,  however, 
suffers  under  the  disadvantage  of  expensiveness.2 

The  other  reagent  is  perchloric  acid,  potassium  perchlorate 

1  The  term  "  platinichloride  "  is  used  instead  of  the  more  common  "  chloro- 
platinate  "  because  it  is  in  analogy  with  the  common  names  of  other  double 
salts  of  the  same  or  similar  type;   such  as  the  silicofluorides,  titanofluorides, 
aurichlorides,  platinocyanides,   cobalticyanides,  ferricyanides,  ferrocyanides, 
and  many  others.     As  the  acids  of  these  are  not  oxy-acids,  and  do  not  contain 
an  element?  that  replaces  oxygen  (as  in  the  thiocyanates),  the  termination 
-ate  is  not  appropriate.     "  Chloroplatinate  "  and  the  names  of  similar  salts 
of  other  such  acids  of  the  platinum  group  are  anomalous,  and  would  seem  to 
have  no  excuse  for  being  except  custom  and  euphony.     "  Platinichloride  "  is 
the  form  used  by  the  Chemical  Society  of  London. 

2  Winter  (Jour.  Ind.  Eng.  Chem.,  8,  p.  87,  1916)  calculated  from  the  loss  in 
unrecovered  platinum  and  the  cost  of  recovery  after  670  determinations  that 
the  cost  of  each  was  $0.0194  (less  than  two  cents),  the  price  of  platinum 
being  reckoned  at  $1.80  per  gram. 


POTASH  AND  SODA  203 

being  practically  insoluble,  and  sodium  perchlorate  easily  soluble, 
in  strong  alcohol  containing  a  little  perchloric  acid. 

The  perchlorate  method  is  said  to  be  almost,  if  not  quite,  as 
accurate  as  the  platinichloride  method.  But  as  I  have  used  the 
latter  exclusively  it  will  here  be  described  in  detail,  and  the  per- 
chlorate method  will  be  more  briefly  treated.1 

Separation  as  Platinichloride. — The  dried  and  weighed  chlo- 
rides as  obtained  above  (p.  202)  are  dissolved  in  the  crucible 
with  about  5  c.c.  of  water,  and  to  this,  the  requisite  amount  of 
a  solution  of  chloroplatinic  acid  is  added. 

While  it  is  necessary  to  add  more  than  enough  of  this  reagent 
to  change  the  entire  amount  of  both  sodium  and  potassium  chlo- 
rides into  platinichlorides,  yet  any  large  excess  is  to  be  avoided, 
on  account  of  the  high  cost  of  chloroplatinic  acid,  if  for  no  other 
reason.  We  therefore  use  a  solution  of  chloroplatinic  acid  made 
up  to  contain  0.05  gram  of  platinum  to  the  cubic  centimeter,  as 
described  elsewhere  (p.  50).  As  it  will  take  3.36  c.c.  of  this  to 
react  completely  with  0.1  gram  of  NaCl  to  form  Na2PtCl6,  and 
only  2.62  to  do  the  same  with  KC1  to  form  K^PtCle,  and  as  nearly 
all  rocks  contain  both  alkalies,  we  are  sure  of  an  excess  if  we 
assume  that  the  chlorides  are  wholly  sodium  chloride,  and  calculate 
the  amount  of  chloroplatinic  acid  solution  used  on  this  basis. 
We,  therefore,  multiply  the  weight  of  the  combined  chlorides  by  34, 
and  the  result  will  be  the  number  of  cubic  centimeters  of  platinum 
solution  which  is  to  be  added.  If  the  rock  is  extremely  rich  in  sodic 
minerals,  as  albite  or  nephelite,  with  little  or  no  potash,  it  will  be 
well  to  take  a  few  drops  more  than  this.2 

The  crucible  is  then  placed  on  the  water-bath  and  heated,  the 
water  being  allowed  only  to  simmer,  or  attain  at  most  a  very 
gentle  boiling,  to  avoid  any  dehydration  of  the  sodium  platini- 
chloride, although  I  have  never  observed  this  to  happen,  even 
with  a  fairly  brisk  boiling.  If  the  precipitated  potassium  platini- 
chloride does  not  wholly  dissolve  when  the  liquid  has  become 

1  For  the  determination  of  the  potash  alone  by  the  cobaltic  nitrite  method 
the  student  may  consult  Hillebrand,  Bull.  422,  p.  177;    and  Mellor,  p.  540. 
I  have  never  tried  the  method  and  therefore  can  not  recommend  it.     See 
p.  208. 

2  The  calculation  need  be  only  approximate.     Thus,  if  the  chlorides  weigh 
.1273  gram  we  say  .13X34=4.42  c.c.,  and  use  a  trifle  more  than  5£  c.c.  of  the 
platinum  solution. 


204 F  METHODS 

warm,  a  few  cubic  centimeters  of  water  are  to  be  added  to  effect 
its  solution.  This  will  seldom  be  necessary  if  the  directions  and 
strengths  of  solutions  given  above  are  followed,  even  with  highly 
potassic,  leucite  rocks. 

The  contents  of  the  crucible  are  evaporated,  with  occasional 
slight  shaking,  to  break  up  the  crust  as  it  forms,  till  the  liquid  is 
syrupy  and  the  mass  solidifies  on  cooling.  This  will  take  place 
when  the  depth  of  the  liquid  is  reduced  to  about  2  mm.,  but  is 
naturally  dependent  on  the  amount  of  alkalies  in  the  rock.  The 
evaporation  should  never  be  carried  to  complete  dryness  on  the 
water-bath,  as  partial  dehydration  of  the  sodium  salt  will  occur, 
the  anhydrous  sodium  platinichloride  being  soluble  with  some 
difficulty  in  alcohol,  and  thus  possibly  adding  to  the  apparent 
amount  of  potassium. 

When  the  evaporation  is  finished  the  crucible  is  removed, 
covered,  and  allowed  to  cool,  so  as  to  make  sure  that  the  liquid 
solidifies.  It  is  then  half  filled  with  alcohol  of  0.86  specific  gravity,1 
contained  in  a  small  wash-bottle,  and  allowed  to  soak.  During 
this  the  Gooch  crucible  is  prepared  with  an  asbestos  felt,  as 
already  described  (p.  99),  ignited,  cooled  and  weighed,  and 
placed  in  position  in  the  filtering  flask.2 

By  this  time  the  disintegration  of  the  solid  mass  in  the  crucible 
should  be  complete.  If  not,  it  may  be  hastened  by  stirring  and 
rubbing  cautiously  with  the  lower  end  of  a  5-c.c.  pipette,  the 
lower  aperture  of  which  should  be  from  1  to  2  mm.  wide.  When 
solution  is  complete,  except  for  the  precipitated,  golden-yellow 
crystals  of  potassium  platinichloride,3  the  suction  is  started 

1  If  a  hydrometer  is  not  at  hand  an  alcohol  of  approximately  this  specific 
gravity  may  be  made  by  mixing  five  volumes  of  ordinary  95  per  cent  alcohol 
with  one  volume  of  water.     Morozewicz  (Bull.  Acad.  Sci.  Crac.,  p.  796,  1906), 
has  shown  that,  when  Na2PtCl6  is  present,  alcohol  stronger  than  80  per  cent 
decomposes  the  sodium  salt,  precipitating  sodium  chloride.     An  alcohol  of 
less  strength  than  75  per  cent  dissolves  appreciable  amounts  of  K2PtCl6  (cf. 
Mellor,  p.  231). 

2  If  this  has  been  previously  used  for  the  determination  of  magnesia,  it, 
as  well  as  the  carbon  filter  and  rubber,  must  be  thoroughly  washed  free  from 
all  traces  of  ammonia. 

3  If  the  fluid  is  not  yellow,  or  if  small  white  grains  (of  sodium  chloride) 
are  present  among  the  yellow  crystals  of  K2PtCl6,  there  has  not  been  enough 
platinum  solution  added.     About  1  c.c.  and  a  little  water  are  to  be  added 
and  the  liquid  again  evaporated  nearly  to  dryness. 


POTASH  AND  SODA  205 

beneath  the  Gooch  crucible  and  the  fluid  is  transferred  to  it  by 
means  of  the  pipette.  The  crucible  with  the  liquid  is  held  in 
the  left  hand  close  to  the  Gooch,  a  little  liquid  sucked  up  into  the 
pipette  and  allowed  to  run  down  the  sides  of  the  filter,  to  avoid 
breaking  the  felt.  When  all  the  liquid  has  been  thus  decanted,  a 
little  more  alcohol  is  poured  on  the  precipitate  in  the  crucible,  and 
decanted  as  before  into  the  Gooch  when  this  is  empty.  After 
three  or  four  decantations,  by  which  time  the  soluble  salts  are 
nearly  gone  and  the  liquid  is  almost  colorless,  the  sides  of  the 
Gooch  crucible  are  carefully  washed  down  with  a  stream  of  alcohol 
and  the  pipette  is  rinsed  both  inside  and  out  into  the  Gooch,  which 
is  filled  with  alcohol  to  wash  the  rim.  The  bulk  of  the  precipitate 
is  then  transferred  to  the  filter,  without  the  use  of  a  rod,  by  a 
gentle  stream  from  the  alcohol  wash-bottle,  the  depth  of  liquid 
being  so  great  that  the  drops  can  fall  in  the  center  without  danger 
of  breaking  the  felt. 

With  a  slender  stirring-rod  capped  with  a  bit  of  fine  rubber 
tubing  the  small  quantity  of  adhering  potassium  platinichloride 
is  loosened  and  is  washed  into  the  filter,  the  stirring-rod  being 
also  rinsed  off.  Owing  to  the  bright  color  and  the  high  specific 
gravity  of  the  precipitate,  it  is  easy  to  be  sure  of  its  complete 
transfer.  When  the  Gooch  is  again  empty  it  is  well  washed,  at 
least  half  a  dozen  times,  the  sides  being  also  washed  down,  inside 
and  out.  Enough  alcohol  may  be  added  each  time  to  half  fill  the 
crucible,  but  it  must  be  allowed  to  empty  before  another  addition. 
After  washing  for  the  last  time,  aspiration  is  continued  for  a  few 
minutes  to  partially  dry  the  felt. 

The  final  drying  is  accomplished  in  an  air-bath  at  a  tempera- 
ture of  130°,  which  is  necessary  to  drive  off  all  the  water.  The 
bottom  cap  of  the  Gooch  crucible  is  placed  in  position,  and  the 
crucible  is  covered  while  in  the  air-bath  with  a  7-cm.  filter-paper 
instead  of  the  cover.  This  permits  evaporation,  and  at  the 
same  time  guards  against  particles  falling  in  from  the  top  of  the 
air-bath.  The  drying  will  usually  be  complete  in  half  an  hour, 
but  it  is  as  well  after  heating  for  this  time  and  having  been  weighed, 
to  reheat  for  another  fifteen  minutes,  or  to  constant  weight. 
After  cooling  in  the  desiccator  the  Gooch  crucible  is  weighed, 
and  recorded  as  Gooch +K2PtClo.  The  weight  of  the  potassium 


206  METHODS 

platinichloride  is  multiplied  by  0.19381  to  arrive  at  the  weight  of 
K2O,  from  which  is  to  be  subtracted  the  amount  of  K20  present 
in  4  grams  of  the  calcium  carbonate  used,  if  this  has  been  deter- 
mined. 

The  weight  of  K2PtCl6  is  then  multiplied  by  0.307  to  reduce  it 
to  KC1,  and  the  weight  thus  obtained  is  deducted  from  that  of 
the  mixed  chlorides.  The  weight  of  the  NaCl  thus  obtained  is 
multiplied  by  0.5308  to  reduce  it  to  Na20,  which  is  to  be  cor- 
rected for  the  amount  of  Na20  present  in  the  calcium  carbonate. 

If  a  Gooch  crucible  is  not  available  the  method  suggested  by 
Hillebrand  may  be  adopted.  This  consists  in  filtering  off  the 
excess  of  chloroplatinic  acid  solution  through  a  small  filter  (5| 
cm.)  and  washing  with  alcohol,  as  little  of  the  precipitate  as 
possible  being  brought  on  the  filter.  When  the  precipitate  has 
been  washed  free  from  all  soluble  matter  that  which  is  on  the  filter 
is  washed  into  the  weighed  crucible  by  small  amounts  of  hot  water, 
the  excess  of  liquid  is  evaporated  to  dry  ness  on  the  water-bath, 
and  the  precipitate  is  finally  dried  as  above  at  130°.  Hille- 
brand prefers  the  use  of  porcelain  for  the  evaporation  of  the 
alcoholic  platinum  solution,  but  for  most  work  this  is  hardly 
necessary. 

It  is  seen  that  the  amount  of  Na2O  is  determined  by  differ- 
ence. But,  in  view  of  the  accuracy  of  the  method,  this  is  prefer- 
able to  a  direct  determination  in  the  filtrate.  If  it  is  desired  to 
do  this,  the  filtrate  is  to  be  freed  from  platinum  by  one  of  the 
methods  recommended  by  Hillebrand,2  and  the  sodium  is  deter- 
mined as  sulphate  in  the  usual  way,  by  evaporation  with  sulphuric 
acid. 

There  is  scarcely  ever  enough  lithium  present  in  igneous  rocks 
to  warrant  its  quantitative  estimation.  It  is  generally  present 
in  spectroscopic  traces,  but,  so  far,  there  seems  to  be  no  theoretical 
necessity  of  establishing  this  fact  in  every  rock  analysis.  If  it  is 
desired  to  do  this,  the  filtrate  from  the  potassium  platinichloride 
is  to  be  evaporated  to  dryness  and  tested  with  the  spectroscope. 
If  it  be  desired  to  estimate  it  quantitatively,  Hillebrand's  direc- 
tions and  his  summary  of  Gooch's  method  are  to  be  followed.3 

1  For  a  discussion  of  the  proper  factors  see  Mellor,  pp.  233,  250. 

2  Hillebrand,  p.  174. 

3  Ibid.,  p.  175. 


POTASH  AND  SODA  207 

All  the  platinum  residues,  both  the  platinichloride  in  the 
Gooch  crucible  and  the  nitrates,  should  be  preserved  in  a  wide- 
mouth  bottle.  The  platinum  may  be  recovered  from  time  to  time 
by  the  usual  methods.1 

Separation  as  Perchlorate. — As  I  have  not  personally  tested 
this  method,  which  has  recently  come  into  prominence  because  of 
the  high  cost  of  platinum,  I  shall  base  the  brief  description  on  the 
authorities  cited  below.2 

The  alkalies  are  obtained  in  the  form  of  mixed  chlorides,  free 
from  ammonium  chloride,  by  the  Smith  method  described  above, 
so  that  we  start  with  these  in  the  platinum  crucible  after  weighing 
(p.  202). 

The  chlorides  are  dissolved  in  the  crucible  in  10  to  20  c.c.  of 
water,  and  more  than  enough  perchloric  acid  3  solution  added  to 
combine  with  the  chlorides.  About  1  c.c.  of  20  per  cent,  or 
0.7  c.c.  of  30  per  cent  acid  for  each  0.1  gram  of  mixed  chlorides  will 
suffice.  The  liquid  is  then  evaporated  in  the  crucible  until  fumes 
of  perchloric  acid  appear.  Ten  c.c.  of  water  and  a  little  more 
perchloric  acid  are  again  added  and  the  solution  is  evaporated 
until  the  white  fumes  are  seen.4  If  white  fumes  do  not  appear  a 
little  more  water  and  perchloric  acid  are  to  be  added  and  the 
procedure  is  repeated.  The  crystals  should  be  kept  broken  up, 
but  not  reduced  to  a  fine  powder,  during  the  last  evaporation. 

After  cooling,  the  mass  is  treated  with  about  20  c.c.  of  alcohol 
containing  0.2  per  cent  of  perchloric  acid,  and  filtered  through  a 
weighed  Gooch  crucible.  The  perchlorate  crystals  are  washed 
two  or  three  times  by  decantation,  and  three  or  four  times  on  the 
filter,  with  the  0.2  per  cent  solution  of  perchloric  acid  in  alcohol. 
It  is  then  dried  at  130°  for  one-half  to  one  hour  and  weighed,  as 
KC104.  Multiplying  this  weight  by  the  factor  .3176  reduces  it  to 
K20,  and  by  .5956  to  KC1. 

If  the  volume  of  washing  liquid  is  kept  small,  as  can  easily  be 

1  Classen,  2,  p.  262;  Mellor,  p.  240  . 

2Mellor,  p.  237;  Treadwell,  2,  p.  50,  Baxter  and  Kobayashi,  Jour.  Am. 
Chem.  Soc.,  39,  p.  249,  1917;  Gooch  and  Blake,  Am.  Jour.  Sci.,  44,  p.  381, 
1917.  Other  references  will  be  found  in  these  papers. 

3  The  preparation  of  this  is  described  on  p.  52.     Perchloric  acid  of  30 
per  cent  has  a  specific  gravity  of  1.20,  that  of  20  per  cent,  1.12. 

4  It  would  appear  that  for  the  amounts  of  alkalies  found  in  rocks  only  one 
evaporation  will  usually  be  sufficient,  but  it  is  safer  to  evaporate  twice. 


208  METHODS 

done,  "  the  solubility  of  the  precipitated  perchlorate  is  insignificant 
for  practical  purposes." 

Alcoholic  solutions  of  perchloric  acid  are  liable  to  explode  on 
evaporation,  especially  over  a  free  flame.  They  should,  therefore, 
be  allowed  to  evaporate  spontaneously. 

Determination  of  Potash  Alone. — In  technical  work  the  deter- 
mination of  potash  in  silicates  without  regard  to  the  soda,  is  some- 
times called  for.  In  this  case  \  gram  of  the  rock  powder  is 
decomposed  by  a  mixture  of  dilute  (1:1)  sulphuric  and  hydro- 
fluoric acids,  and  the  hydrofluoric  acid  is  driven  off  by  several 
evaporations.  The  potash  may  then  be  separated  by  the  sodium 
cobaltinitrite  method,1  and  determined  either  as  platinichloride 
or  as  perchlorate. 

A  much  more  rapid  method,  and  one  giving  very  accurate 
results,  is  that  suggested  by  Hicks  and  Bailey,2  and  now  used  in 
the  U.  S.  Geological  Survey  laboratory  for  the  determination  of 
potash  in  glauconite  and  such  materials.  I  have  had  experience 
with  it,  with  satisfactory  results. 

After  decomposition  with  sulphuric  and  hydrofluoric  acids, 
the  dried  residue  is  dissolved  in  dilute  hydrochloric  acid.  After 
filtering,  this  solution  is  evaporated  to  dryness  with  the  addition 
of  a  slight  excess  of  chloroplatinic  acid.  The  residue  is  washed 
on  a  small  paper  filter  with  alcohol  containing  2  per  cent  of  con- 
centrated hydrochloric  acid  until  free  from  excess  of  chloroplatinic 
acid  and  platinichlorides.  The  potassium  platinichloride  is 
washed  through  the  filter  with  hot  water,  the  platinichloride 
reduced  to  metallic  platinum  by  magnesium,  and  the  platinum  is 
weighed.  Multiplication  of  the  weight  of  platinum  by  0.4826 
gives  the  weight  of  K2O.  For  details  the  original  paper  may  be 
consulted. 

14.  HYGROSCOPIC  WATER  3 

By  this  term  is  meant  the  moisture  which  is  adsorbed  by  the 
rock  powder  from  the  atmosphere,  or  which  may  come  from  that 
enclosed  in  microscopic  cavities,  and  a  part  of  the  more  loosely 
combined  water  of  crystallization  of  some  zeolites  and  other 

1  Cf.  Hillebrand,  Bull.  422,  p.  177;   Mellor,  p.  540. 

2  Hicks  and  Bailey,  m  U.  S.  Geol.  Surv.,  Bull.  660-B,  p.  53,  1917. 

3  Fresenius,  1,  p.  74;  Hillebrand,  pp.  69-70;  Mellor,  pp.  155-157. 


HYGROSCOPIC  WATER  209 

hydrous  minerals  may  'also  be  included  under  this  head.  It  is  all, 
or  nearly  all,  expelled  from  the  rock  at  temperatures  of  about  110°, 
though  Day  and  Allen  1  conclude  from  their  study  of  feldspar 
powders  that  some  of  it  is  not  expelled  below  600°  to  800°.  The 
amount  of  hygroscopic  water  in  fresh  rocks  is  usually  very  small, 
and  Day  and  Allen  1  note  that  the  quantities  of  adsorbed  water 
determined  by  them  "  are  of  the  same  order  of  magnitude  as 
those  usually  obtained  for  the  water  content  in  feldspar  analyses." 
Notwithstanding  this,  it  is  always  well  to  determine  hygroscopic 
water  separately  from  combined  water.  The  reasons  for  this 
have  been  fully  discussed  by  Hillebrand  2  and  need  not  be  gone 
into  here. 

The  influence  of  the  fineness  of  the  powder  on  the  adsorp- 
tion of  moisture  from  the  atmosphere,  and  the  consequent  advisa- 
bility of  analyzing  air-dry  material,  have  already  been  mentioned 
(pp.  66,  72). 

About  1  gram  of  the  rock  powder  is  weighed  out  into  a  pre- 
viously ignited  and  cooled  platinum  crucible  of  30  or  40  c.c. 
capacity  and  this  is  heated  in  an  air-bath  at  a  temperature  a  little 
above  that  of  boiling  water.  The  exact  temperature  is  of  no  great 
importance,  as  long  as  it  is  slightly  above  100°.  In  the  U.  S. 
Geological  Survey  laboratory  a  toluene  bath  is  used,  giving  a  tem- 
perature of  105°  (Hillebrand),  while  my  practice  is  to  use  an 
ordinary  copper  air-bath,  with  single  walls,  and  the  flame  so  reg- 
ulated as  to  maintain  the  temperature  constantly  at  110°,  which 
is  readily  accomplished.  The  crucible  is  covered  during  the  heat- 
ing with  a  7-cm.  filter-paper,  the  platinum  cover  being  removed. 
It  will  usually  be  found  that  half  an  hour's  heating,  and  often  less, 
will  be  sufficient  to  arrive  at  a  constant  weight.  After  heating, 
the  crucible  is  allowed  to  cool  in  a  desiccator  and  is  weighed, 
heated  again  for  a  quarter  of  an  hour,  and  if  the  weight  is  constant, 
the  loss  in  weight,  divided  by  the  weight  of  rock  powder  taken, 
gives  the  percentage  of  hygroscopic  water,  which  may  be  conveni- 
ently tabulated  as  H20  — . 

The  portion  used  for  this  determination  may  be  used  for  that 
of  any  other  constituent,  except  alkalies  and  ferrous  oxide.  Indeed, 

1  Day  and  Allen,  Isomorphism  of  the  Felsdpars.     Carnegie  Publication 
No.  31,  p.  57,  1905. 

2  Hillebrand,  pp.  57-70. 


210  METHODS 

it  is  often  a  saving  of  time  if  this  be  done.  Determinations  that 
suggest  themselves  are  those  for  the  main  sodium  carbonate 
fusion,  for  manganese,  and  for  baryta  and  zirconia. 

15.  COMBINED  WATER  1 

Under  this  head  is  included  all  the  water  in  a  rock  which  is 
chemically  combined,  in  mineral  molecules,  either  as  water  of 
crystallization  (as  in  analcite)  or  as  hydroxyl  (as  in  muscovite  or 
biotite) . 

Few  constituents  of  rocks  have  had  the  amount  of  time, 
thought,  labor,  and  ingenuity  in  devising  apparatus,  expended  on 
them  that  have  been  devoted  to  the  accurate  determination  of 
combined  water;  and,  in  all  the  text-books,  an  amount  of  space 
and  attention  are  devoted  to  it  that  is  quite  disproportionate  with 
its  importance  in  rock  analysis. 

In  the  analysis  of  minerals,  in  many  of  which  the  water  or 
hydroxyl  may  play  a  very  important  part  in  the  constitution  of 
the  molecule,  the  accurate  determination  of  water  may  be  (and 
often  is)  of  very  great  importance  for  the  correct  interpretation 
of  the  composition  and  molecular  constitution  of  the  mineral. 
In  fresh  rocks,  on  the  other  hand,  water  is  usually  a  constituent  of 
very  subsidiary  and  minor  importance;  so  that  the  use  of  any  of 
the  very  complicated  forms  of  apparatus  that  have  been  devised 
is  quite  uncalled  for,  unless  there  is  some  special  reason  for  going 
to  all  the  trouble  that  they  involve.  Even  with  such  rocks,  the 
simple  Penfield  method,  which  is  capable  of  great  accuracy,  will 
usually  be  found  to  be  entirely  adequate  to  the  purpose. 

Errors. — Because  of  its  usually  minor  importance  in  the  anal- 
ysis of  rocks,  the  errors  involved  in  the  determination  of  water  will 
seldom  be  serious. 

If  the  water  is  determined  by  loss  on  ignition,  a  plus  error  will 
be  introduced  by  the  expulsion  of  other  volatilizable  substances 
such  as  carbon  dioxide,  sulphur,  or  chlorine.  Only  that  of  the  car- 
bon dioxide  is  likely  to  be  of  serious  consequence.  The  "  hygro- 
scopic "  water  will,  of  course,  also  be  expelled,  and  should  be 

1  Classen,  2,  pp.  626-634;  Fresenius,  1,  pp.  72-78;  Gooch,  pp.  34-38; 
Hillebrand,  Bull.  422,  pp.  57-83;  Mellor,  pp.  157-159;  570-575;  Treadwell,  2, 
p.  512;  S.  L.  Penfield,  Am.  Jour.  Sci.,  48,  pp.  30-37,  1894. 


COMBINED  WATER  211 

allowed  for.  A  minus  error  is  introduced  by  oxidation  of  any 
ferrous  oxide  present;  this  may  be  so  great  that,  if  the  rock  con- 
tains much  ferrous  oxide  and  but  little  water  (as  may  happen 
with  basalts,  for  instance),  there  will  be  an  actual  gain,  instead  of 
loss,  in  weight  on  ignition. 

If  the  water  is  determined  by  an  absorption  method,  presence 
of  moisture  in  the  air  used  to  pass  through  the  apparatus  will  add 
to,  and  failure  to  expel  or  absorb  all  of  the  water  will  lessen,  its 
apparent  amount. 

In  the  Penfield  method  the  only  notable  source  of  error  in 
general  is  that  due  to  the  presence  of  carbon  dioxide  in  the  sub- 
stance. Unless  this  is  allowed  to  escape  from  the  tube  that  con- 
tains the  water,  its  higher  .specific  gravity  as  compared  with  that 
of  air  will  diminish  the  apparent  weight  of  water.  A  correction 
may  be  made  for  it,  and  Penfield's  paper  should  be  consulted  on 
this  point.  For  fresh  rocks  the  error  will  be  of  no  consequence. 

In  all  water  determinations  in  rocks  or  minerals,  regard  must 
be  had  for  the  very  high  temperature  at  which  the  hydroxyl  is 
completely  driven  off  (in  the  form  of  water)  from  some  minerals, 
as  with  talc,  topaz,  chondrodite,  or  staurolite.  For  very  exact 
determinations  this  point  must  be  looked  into,  so  that,  if  necessary, 
a  sufficiently  high  temperature  is  used  for  complete  expulsion. 

Loss  on  Ignition. — The  early  method,  and  one  very  frequently 
used,  even  at  the  present  day,  for  the  determination  of  this  com- 
bined water,  is  that  of  simple  ignition  in  a  platinum  crucible,  the 
assumption  being  that  this  "  loss  on  ignition  "  represents  only 
the  total  water  in  the  rock.  A  little  consideration  shows  that 
the  results  under  these  circumstances  will  only  be  accurate  when 
the  rock  contains  neither  substances  which  are  easily  volatilizable 
at  the  temperature  of  ignition  (as  carbon  dioxide,  carbon  and 
organic  matter,  sulphur,  chlorine,  or  fluorine)  nor  oxidizable 
constituents  (as  ferrous  oxide).  In  the  former  case  the  apparent 
amount  of  water  will  be  too  great,  owing  to  the  partial  or  entire 
loss  of  the  volatilizable  ingredients,  and  in  the  latter  it  will  be  too 
small,  on  account  of  the  gain  in  weight  through  the  oxidation  of 
ferrous  oxide  to  ferric. 

It  is  held  by  some  that  the  error  due  to  the  latter  cause  may 
be  corrected  by  calculation  of  the  gain  in  weight  which  the  fer- 
rous oxide  that  is  present  in  the  rock,  and  which  is  separately  deter- 


212  METHODS 

mined,  would  undergo  if  completely  oxidized  to  ferric  oxide. 
This  assumption,  however,  is  by  no  means  valid  under  the  cir- 
cumstances obtaining  in  the  process  of  ignition,1  as  is  shown,  for 
example,  by  the  difficulty  of  completely  oxidizing  magnetite  by 
ordinary  ignition,  even  after  roasting  with  nitric  acid. 

In  the  case  of  volatilizable  constituents,  also,  there  can  scarcely 
ever  be  a  certainty  that  their  loss  in  this  way  will  be  complete,  so 
that  appropriate  corrections  may  be  made  with  safety  after  their 
separate  determination.  This  would  be  true  only  of  carbon  dioxide 
when  derived  from  calcite,  magnesite,  or  dolomite,  and  then  only 
after  prolonged  blasting. 

This  being  so,  and  it  being  also  a  fact  that  there  are  few  rocks 
which  contain  no  such  disturbing  constituents  (especially  FeO), 
it  follows  that  with  the  great  majority  the  combined  water  should 
not  be  determined  by  loss  on  ignition. 

As,  however,  the  determination  of  combined  water  is  not 
always  of  great  importance  for  the  chemical  study  of  rocks,  it 
happens  that  this  simple  method  may  be  used  in  certain  cases. 
These  would  include  very  fresh  igneous  rocks,  that  contain  but 
a  small  amount  of  water,  no  other  volatilizable  ingredients, 
and  only  a  small  amount  of  ferromagnesian  minerals,  say  up 
to  5  per  cent,  and  consequently  only  1  or  2  per  cent  of  ferrous 
oxide.  Many  granites,  porphyries,  syenites,  trachytes,  rhyolites, 
andesites,  and  anorthosites  fall  under  this  description.  For  such 
rocks  the  minute  error  due  to  the  very  small  amount  of  ferrous 
oxide  present  (amounting  at  most  to  one-ninth  of  its  weight)  may 
be  deemed  to  be  negligible,  and  the  results  of  such  a  determination 
may  be  regarded  as  acceptable. 

If  the  method  of  "  loss  on  ignition  "  is  to  be  employed,  the 
weighed  crucible  and  its  contents,  which  have  previously  been 
used  for  the  determination  of  hygroscopic  water,2  are  ignited 
(covered)  at  a  bright-red  heat  for  about  half  an  hour,  or  to  con- 
stant weight,  cooled  in  the  desiccator  and  weighed.  The  loss  in 
weight  represents  the  amount  of  combined  water.  The  fact  must, 

1  Cf.  Sosman  and  Hostetter,  Jour.  Am.  Chem.  Soc.,  38,  p.  820,   1916; 
Trans.  Am.  Inst.  Min.  Eng.,  p.  907,  1917. 

2  It  is  not  advisable  to  use  a  portion  in  which  any  other  than  hygroscopic 
water  is  determined,  as  the  rock  powder  is  apt  to  sinter  through  the  great  heat 
and  be  attacked  with  difficulty  by  fluxes.     A  separate  portion  may  be  used. 


COMBINED  WATER  213 

however,  be  recognized  that  this  method  of  procedure  is  not  strictly 
accurate,  and  that  for  all  high-class  work,  and  in  all  rocks  where  the 
amount  of  ferrous  oxide  is  at  all  considerable,  or,  if  volatilizable 
substances  are  present,  the  combined  water  should  be  determined 
directly. 

Penfield's  Method. — The  general  inadvisability  of  employing 
any  of  the  more  complex  methods  spoken  of  above,  and  some  of 
which  are  described  in  the  authors  cited,  has  already  been  men- 
tioned. Fortunately,  a  very  simple  and  accurate  method  has  been 
devised  by  Penfield,1  which  meets  all  the  requirements  with  the 
great  majority  of  rocks,  needs  no  elaborate  apparatus,  and  takes 
but  about  half  an  hour  for  its  execution.  It  would  seem  to  be 
little  known,2  much  less  than  it  deserves  to  be. 

This  consists  in  igniting  the  rock  powder  in  a  tube  of  hard  glass, 
closed  at  one  end  and  with  or  without  enlargements  in  the  middle, 
pulling  off  the  heated  end  containing  the  powder,  weighing  the 
portion  of  the  tube  which  contains  the  expelled  water,  and  finally 
weighing  this  portion  of  the  tube  after  thorough  drying.  This 
gives  the  total  amount  of  water,  hygroscopic  and  combined, 
from  which  the  amount  of  the  former,  as  previously  obtained, 
is  to  be  deducted  to  obtain  the  latter.  For  illustrations  of  the 
apparatus  used  the  reader  is  referred  to  the  paper  cited  above. 

With  most  fresh  igneous  rocks  a  simple  tube  of  rather  hard 
glass  is  used,  closed  at  one  end,  and  without  any  enlargement. 
The  dimensions  recommended  by  Penfield  are  20  to  25  cm.  long  3 
and  with  an  internal  diameter  of  about  6  mm.  If  the  rock  con- 
tains more  than  a  fraction  of  a  per  cent  of  water  it  is  better  to  have 
a  bulb  or  enlargement  blown  about  midway  in  the  tube.  Indeed, 
this  is  always  advisable,  to  guard  against  drops  of  water  rolling 
back  on  the  heated  portion.  A  single  bulb  is  sufficient  for  nearly 
all  rocks,  and  the  more  complicated  forms  illustrated  by  Penfield 
will  seldom  be  found  necessary  in  rock  analysis,  unless  the  rock 
is  not  fresh. 

1  S.  L.  Penfield,  Am.  Jour.  Sci.,  48,  p.  30,  1894. 

2  Neither  Classen,  Fresenius,  Gooch,  Mellor  nor  Morse,  mention  it;  while 
Hillebrand  and  Treadwell  give  but  very  brief  descriptions. 

3  The  tube  must  not  be  too  long  to  go  in  the  balance-case,  and  so  inter- 
fere with  weighing,  nor  too  short,  so  as  to  give  rise  to  the  danger  of  loss  of 
water  through  lack  of  sufficient  cooling  surface  and  heating  of  the  cooler 
portion. 


214  METHODS 

It  is  of  importance  to  have  the  tube  thoroughly  dry,  and  this 
"  is  best  accomplished  by  heating  and  aspirating  a  current  of  air 
through  it  (while  hot)  by  means  of  a  glass  tube  reaching  to  the 
bottom."  This  should  always  be  done,  even  if  the  tube  is  appa- 
rently dry.  After  cooling,  the  tube  is  weighed,  its  weight  including 
that  of  the  brass  tube-support  which  is  used  to  support  it  on  the 
balance-pan. 

From  |  to  1  gram  of  the  rock  powder  is  then  introduced,  filling 
the  tube  about  2  cm.  from  the  closed  end.  This  must  be  done 
without  soiling  the  upper  portion  of  the  tube,  and  is  accomplished 
by  means  of  a  small  thistle-tube,  of  diameter  small  enough  to  slip 
easily  into  the  bulbed  tube,  and  long  enough  to  reach  the  end. 
Such  a  filling-tube  can  be  readily  made  from  a  5-c.c.  pipette  by 
cutting  the  bulb  in  two  and  reducing  the  length  of  the  tube  to 
25  cm.  The  filling-tube,  of  course,  must  also  be  thoroughly  dry.1 
After  the  powder  is  introduced  the  tube  is  weighed  again,  to  obtain 
the  weight  of  substance  used,  the  manipulation  being  delicate  and 
gentle  to  avoid  any  rolling  of  the  powder  toward  the  open  end. 

After  a  few  gentle  taps  to  form  a  free  passage  above  the  powder 
for  the  heated  air,  which  might  otherwise  drive  the  powder  toward 
the  bulb  and  so  necessitate  refilling  and  reweighing,  the  tube  is 
held  in  a  clamp  horizontally,  or  very  slightly  sloping  toward  the 
mouth.  A  narrow  strip  of  filter-paper  or  cloth,  moistened  with 
cold  water  and  kept  moist  and  cool,  is  wrapped  around  the  bulb 
and  the  farther  end  of  the  tube,  so  as  to  ensure  condensation  of 
the  expelled  water,  care  being  taken  that  it  is  not  so  near  the 
mouth  as  to  allow  any  water  dropped  on  it  to  enter  the  tube. 

A  gentle  heat  is  then  applied  to  the  closed  end,  and  is  gradually 
increased  to  the  full  heat  of  the  Bunsen  burner.  The  blast 
may  be  used  if  minerals  are  known  to  be  present  which  only 
give  off  their  water  with  difficulty,  but  this  will  not  be  needed  in 
most  rocks.  If  the  strip  of  cloth  or  filter-paper  be  kept  moist, 
there  is  scarcely  need  for  a  screen  of  asbestos  board,  nor  is  it 
often  necessary  to  partially  close  the  tube  with  another  short 
piece  of  tube  drawn  out  to  a  capillary  and  connected  by  rubber 
tubing.  If  the  heated  end  of  the  tube  tends  to  sink,  this  should 

1  The  thistle-tube  can  be  easily  cleaned  "  by  drawing  through  it  a  bit 
of  cotton  attached  to  a  wire,"  or,  if  the  analyst  be  a  smoker,  a  fresh-pipe- 
cleaner  will  answer  admirably. 


COMBINED  WATER  215 

be  remedied  by  gently  turning  it  around  from  time  to  time  parallel  to 
its  axis,  the  clamp  being  adjusted  so  as  to  allow  of  this  being  done. 

After  the  whole  extent  of  the  powder  has  been  ignited  and  the 
water  completely  expelled,  which  will  take  at  least  a  quarter  of  an 
hour,  a  short  piece  of  narrow  tubing  is  melted  onto  the  closed  tip, 
to  serve  as  a  handle.  The  flame  is  then  lowered  and  the  water 
is  very  gently  and  gradually  driven  into  the  bulb.  This  must  be 
carried  out  with  caution  and  patience  to  avoid  cracking  the  tube. 
When  the  water  has  been  driven  into  the  bulb  and  to  a  safe  dis- 
tance, the  portion  of  the  tube  immediately  in  front  of  the  powder 
is  heated  to  softness  all  around,  and  the  end  containing  the  powder 
drawn  off  and  the  other  part  sealed  without  allowing  the  flame 
to  enter.  The  tip  of  a  Meker  burner  flame  serves  well  for  this. 

The  remaining  portion  of  the  tube  containing  the  water  is 
allowed  to  cool  in  the  clamp  in  a  horizontal  position,  is  wiped  clean 
and  dry  on  the  outside  and  weighed.  It  is  well  to  test  the  water 
with  blue  and  red  litmus-paper  after  weighing.  It  is  then  placed 
again  in  the  clamp  and  gently  heated,  the  moist  air  and  steam 
being  sucked  out  by  means  of  a  small  tube  extending  to  the  bottom 
and  connected  with  a  suction-pump.  After  thorough  drying  in 
this  way  it  is  allowed  to  cool  and  is  again  weighed. 

The  loss  in  weight  is  the  amount  of  total  water,  which  is 
reduced  to  percentage  figures  by  division  by  the  amount  of  sub- 
stance taken;  the  percentage  of  hygroscopic  water  already  deter- 
mined is  then  subtracted. 

In  nearly  all  cases  the  simple  method  described  above  will  be 
quite  sufficient  and  will  yield  very  accurate  results.  But  when 
rocks,  such  as  some  metamorphic  ones,  contain  minerals  like 
talc,  topaz,  chondrodite,  or  staurolite,  whose  water  is  not  com- 
pletely driven  off  over  the  blast,  it  becomes  necessary  to  use  a 
more  intense  method  of  heating.  For  a  description  of  this,  refer- 
ence may  be  made  to  Penfield's  article. 

If  the  rock  contains  constituents  like  SOs,  S,  Cl  or  F  in  appre- 
ciable amount,  which  are  volatile  and  which  will  add  to  the 
weight  of  the  water  driven  off  and  condensed,  it  is  necessary  to 
use  a  retainer  for  these  during  the  ignition.  The  best  of  these 
is  lime,  previously  ignited  and  cooled.1  A  little  of  this  is  intro- 

1  Penfield  and  Howe  (Am.  Jour.  Sci.,  47,  p.  191, 1894)  state  that  lead  oxide 
which  is  often  used,  is  wholly  unsuitable. 


216  METHODS 

duced  by  means  of  the  thistle-tube  into  the  bulbed  tube,  after  the 
rock  powder  has  been  weighed,  and  mixed  "  by  means  of  a  fine  wire 
bent  into  a  corkscrew  coil  at  the  end."  A  decigram  or  two  will  be 
ample  for  most  rocks.  In  ordinary  rock  analysis  the  correction  for 
C02,  described  by  Penfield,  will  not  be  necessary. 

16.  PHOSPHORUS  PENToxiDE1 

As  the  amount  of  material  is  usually  ample  in  rock  analysis, 
it  is  best  to  determine  phosphorus  pentoxide  in  a  separate  por- 
tion of  rock  powder,  although  it  can  be  determined  in  the  solution 
used  for  the  total  iron  oxides  and  titanium  dioxide,  as  mentioned 
below. 

Errors. — The  chief  errors  to  which  the  determination  of  phos- 
phorus pentoxide  is  subject  are  caused  by  the  somewhat  uncertain 
compositions  of  the  ammonium  phosphomolybdate  and  ammonium 
magnesium  phosphate  precipitates,  which  vary  according  to  the 
conditions  that  obtain  during  precipitation. 

It  is  not  necessary  to  go  into  the  subject  here,  and  the  student 
will  find  details  given  by  Mellor.  The  dependence  of  the  compo- 
sition of  the  magnesium  precipitate  on  the  conditions  of  precipi- 
tation has  already  been  touched  on  (p.  180).  It  will  suffice  to 
say  that  the  conditions  laid  down  should  be  followed  closely  to 
obtain  accurate  results.  Fortunately  the  amount  of  phosphorus  in 
igneous  rocks  is  generally  small,  so  that  errors  in  its  determination 
are  of  subsidiary  importance. 

If  considerable  vanadium  is  present  in  the  rock,  a  very  rare 
occurrence,  the  percentage  of  ¥205  is  to  be  subtracted  from  that  of 
?205,  as  the  vanadium  is  precipitated  as  vanadomolybdate  along 
with  the  phosphomolybdate  and  weighed  as  magnesium  vanadate,2 
with  the  magnesium  phosphate  if  phosphorus  is  present  in  much 
greater  quantity  than  the  vanadium,  as  it  invariably  is  in  rocks. 

Precipitation  as  Phosphomolybdate. — The  phosphorus  pentox- 
ide is  best  separated  from  the  other  constituents  present  in  the 
solution  by  the  old  and  well-known  method  of  precipitation  as 

1  Classen,  2,  pp.  564-570;  Fresenius,  1,  pp.  446-447;  Gooch,  pp.  81-82; 
Hillebrand,  Bull.  422,  pp.  144-146;  Mellor,  pp.  590-598;  Treadwell,  2,  pp. 
434-440. 

*  Cain  and  Hostetter,  Tech.  Pap.  Bur.  Stand.,  No.  8,  1912. 


PHOSPHORUS  PENTOXIDE  217 

phosphomolybdate.  The  conditions  recommended  for  this  vary 
considerably,  but  I  have  found  that  Woy's  procedure  is  the  most 
rapid  and  best.1 

For  the  decomposition  about  1  gram  of  rock  powder  is  weighed 
out  into  a  small  platinum  basin,  or,  if  that  is  not  available,  into  a 
capacious  crucible.  The  rock  powder  is  then  mixed  with  10  c.c. 
of  water,  taking  the  precautions  to  prevent  loss  of  powder  noted 
previously,  and  is  stirred  up  with  a  small  platinum  spatula  or 
platinum  wire  which  is  left  in  the  basin  for  stirring.  Ten  c.c.  of 
concentrated  nitric  acid  are  added,  about  5  c.c.  of  hydrofluoric 
acid  are  next  poured  in,  and  the  mixture  is  evaporated  to  dryness 
on  the  water-bath  or  over  a  low  flame  with  occasional  stirring. 
Evaporation  with  small  quantities  of  nitric  acid  alone  is  repeated 
two  or  three  times,  to  decompose  the  fluorides  as  completely  as 
possible. 

When  completely  dry  after  the  last  evaporation,  the  basin 
is  heated  till  its  contents  become  brown,  and  when  cool  the  crust 
•is  moistened  with  5  c.c.  of  a  mixture  of  nitric  acid  diluted  with 
twice  its  bulk  of  water,  and  gently  boiled  for  a  few  minutes, 
to  convert  any  meta-  or  pyro-phosphates,  possibly  produced  by 
the  heating,  into  orthophosphates.  Solution  will  be  complete, 
except  for  the  silica  and  fluorides  present.  The  liquid  is  now  fil- 
tered through  a  5|-cm.  filter  into  a  150-c.c.  beaker.  The  basin  is 
to  be  rinsed  out  and  the  filter  washed  half  a  dozen  times  with  the 
same  warm  dilute  acid.  The  volume  of  the  liquid  should  not  be 
more  than  about  50  c.c. 

About  25  c.c.  of  ammonium  nitrate  solution  (containing  320 
grams  of  ammonium  nitrate  to  the  liter)  are  added,  and  the  liquid 
is  heated  until  it  is  near  boiling.  A  temperature  of  60°-70°  is 
appropriate.  In  the  meantime  25  c.c.2  of  ammonium  molybdate 
solution  (p.  48)  are  heated  to  boiling,  and  poured  in  a  thin  stream 
down  the  stirring-rod  into  the  hot  liquid  in  the  beaker,  which  is 
kept  in  constant  rotation.  Precipitation  is  immediate  and  com- 
plete.3 

1  Mellor,  p.  595;  Treadwell,  2,  p.  436. 

2  If  the  rock  contains  more  than  1  per  cent  of  P2O2  50  c.c.  must  be  used. 

3  The  filtrate  and  washings  from  this  precipitate  should  be  kept  (covered) 
for  a  few  hours.     If  a  yellow  precipitate  forms  all  the  phosphorus  has  not  been 
separated.    The  warmed  filtrate  must  then  be  treated  with  more  ammonium 


218  METHODS 

The  liquid  is  then  filtered  through  another  5|-cm.  filter,  the 
bright-yellow  precipitate  being  disturbed  as  little  as  possible. 
The  latter  is  washed  with  a  mixture  of  weak  solution  of  am- 
monium nitrate,  nitric  acid  and  ammonium  molybdate  solution 
in  equal  parts,  till  the  addition  of  ammonia  water  in  excess  pro- 
duces no  permanent  precipitate  in  a  few  drops  of  the  filtrate  in  a 
watch-glass.  About  50  c.c.  of  the  washing  mixture  will  usually 
be  cmple,  and  it  should  be  prepared  in  a  small  beaker  as 
needed. 

The  phosphorus  is  now  all  in  the  precipitate  of  ammonium 
phosphomolybdate,  and  the  beaker  containing  the  greater  part 
of  this  isjDlaced  beneath  the  funnel,  and  the  filter  is  then  filled  with 
ammonia  water  diluted  with  an  equal  amount  of  water.  This  dis- 
solves the  small  portion  of  precipitate  in  the  filter  and  part  or  the 
whole  of  that  in  the  beaker,  assisted  by  stirring.  If  solution  is 
not  complete  some  more  ammonia  must  be  added.  The  filter  is 
then  washed,  half  a  dozen  fillings  with  water  being  sufficient.1 

If  the  fluid  in  the  beaker  is  turbid,  due  to  the  formation  of  a 
white  compound  of  phosphorus,  as  occasionally  happens,  this 
may  be  overcome  by  the  addition  of  a  small  fragment  of  citric  or 
tartaric  acid.  If  this  fails  to  remove  the  turbidity,  the  liquid  is 
to  be  filtered  through  the  same  filter  into  another  small  beaker, 
the  filter  ignited  in  a  small  platinum  crucible  and  fused  with  a 
pinch  of  sodium  carbonate,  the  small  cake  dissolved  in  water, 
acidified  with  nitric  acid,  and  the  solution  added  to  the  rest 
(Hillebrand).  This  has  never  been  necessary  in  my  experience. 

To  the  solution  in  the  beaker,  which  may  amount  to  50-100 c.c., 
10  c.c.2  of  "  magnesia  mixture"  are  added,  best  through  the  same 
filter,  to  remove  any  possible  deposit  in  the  "  magnesia  mixture." 
The  beaker  is  allowed  to  stand  for  twelve  hours ;  then  the  contents 
are  filtered  through  a  small  filter  and  the  precipitate  collected  on 
the  latter,  that  adhering  to  the  sides  of  the  beaker  being  rubbed 

molybdate  and  this  precipitate  filtered,  washed,  and  treated  like,  anu  added 
to,  the  first. 

1  If  it  has  been  necessary  to  precipitate  more  phosphorus  in  the  previous 
filtrate,  the  two  solutions  of  phosphomolybdate  are,  of  course,  mixed  and 
precipitated  as  above. 

2  If  there  is  much  phosphorus  more  magnesia  mixture  should  be  added. 
At  least  1  c.c.  of  magnesia  mixture  is  needed  for  each  centigram  of  phosphorus 
pentoxide  in  the  solution. 


MANGANOUS  OXIDE  219 

off.     The  filter  and  precipitate  of  ammonium-magnesium  phos- 
phate are  well  washed  with  weak  ammonia  water. 

The  filter  with  its  contents  are  then  placed  in  a  small  weighed 
platinum  crucible,  and,  after  the  filter  has  been  carbonized, 
are  ignited  at  a  bright-red  heat.  When  cool,  the  crucible  and 
contents  are  weighed,  and  the  weight  of  the  Mg2?207  is  multiplied 
by  0.638  to  reduce  it  to  P205.  The  appropriate  weight  of  P2Os 
determined  from  this  percentage  is  to  be  deducted  from  the  weight 
of  the  precipitate  by  ammonia  water  (p.  59),  to  arrive  at  the 
correct  weight  of  alumina. 

The  phosphorus  may  also  be  determined,  more  rapidly  and 
about  as  accurately,  as  phosphomolybdic  anhydride.  For  this 
procedure  the  yellow  phosphomolybdate  precipitate  is  collected  in 
a  weighed  Gooch  crucible  and  washed  with  a  mixture  of  the  ammo- 
nium nitrate  solution  and  dilute  nitric  acid,  about  50  c.c.  being 
usually  sufficient.  The  lower  cap  is  then  put  on,  and  the  crucible 
is  gently  heated  until  the  contents  are  dry  and  no  more  vapors  of 
ammonia  or  ammonium  salts  arise.  The  heat  is  then  raised  until 
the  bottom  is  a  dull  red  and  this  is  continued  until  the  yellow 
mass  is  entirely  decomposed  into  the  greenish-black  anhydride, 
24Mo03.P205.  About  ten  minutes  suffice  for  this.  The  weight 
of  the  anhydride  multiplied  by  0.0395,  or  by  0.04  for  the  usual 
small  amounts,  gives  the  weight  of  P2O5.  According  to  my  experi- 
ence the  method  is  rapid,  accurate,  and  satisfactory. 

If  material  is  scanty  and  it  is  desired  to  determine  phosphorus 
pentoxide  in  the  solution  used  for  total  iron  and  for  titanium  diox- 
ide, the  following  process  will  serve:  The  acid  solution  or  an 
aliquot  portion  of  it,  after  determination  of  titanium  is  precipitated 
with  ammonia,  the  precipitate  washed  with  hot  water  a  few  times, 
dissolved  on  the  filter  with  dilute  nitric  acid  and  the  filter  washed; 
the  filtrate  and  washings  are  evaporated  to  small  bulk,  and  the 
phosphorus  is  precipitated  in  this  by  ammonium  molybdate. 
The  subsequent  operations  are  as  described  above. 

17.  MANGANOUS  OXIDE 

Manganous  oxide  may  be  determined  in  the  main  portion  used 
for  silica,  etc.,  if  the  basic  acetate  method  has  been  used  for  its 
separation.  This  procedure  will  be  described  later  (p.  223). 


220  METHODS 

For  reasons  already  given,  however,  this  method  is  to  be  avoided, 
and  the  manganous  oxide  is  best  determined  in  a  separate  portion 
by  a  colorimetric  method,  which  is  very  accurate  and  serves 
admirably  for  the  small  amounts  (less  than  0.5  per  cent)  that  are 
almost  invariably  found  in  igneous  rocks.  If  the  manganous 
oxide  amounts  to  2  per  cent  or  more,  as  it  may  in  some  silicate 
minerals  or  in  very  exceptional  rocks,  it  is  best  to  separate  it  by 
the  basic  acetate  method  and  determine  it  gravimetrically. 

Errors. — There  are  no  serious  errors  inherent  in  the  colori- 
metric method,  if  the  solution  is  properly  and  completely  oxidized 
to  permanganate,  as  can  be  readily  accomplished  if  the  directions 
given  are  followed. 

The  great  liability  to  error  involved  in  the  basic  acetate  method 
has  already  been  discussed  in  connection  with  alumina  (p.  149). 
The  error  here,  through  weighing  of  alumina  with  the  manganous 
oxide,  may  easily  amount  to  many  times  the  weight  of  the  man- 
ganous oxide  present.  For  this  reason  the  high  figures  often 
reported  for  manganous  oxide  in  rock  analyses  are  regarded  with 
more  than  suspicion.1 

If  manganous  oxide  is  not  separated  and  determined  some  of 
that  present  will  be  precipitated  with  and  appear  as  alumina,  prob- 
ably because  it  is  peroxidized.  A  smaller  portion  falls  with  the 
lime,  and  the  rest — generally  the  largest  portion — is  precipitated 
with  the  magnesia.2  As  the  tamount  of  manganous  oxide  in 
igneous  rocks  is  very  small,  seldom  over  0.50  and  usually  under 
0.20  per  cent,  this  error  will  not  be  serious  for  the  great  majority  of 
rocks  (p.  14). 

Colorimetric  Method.3 — This  method,  introduced  by  Walters, 
is  based  on  the  oxidation  of  manganous  salts  to  permanganates  by 
the  action  of  ammonium  persulphate  in  the  presence  of  silver 
nitrate,  which  acts  as  a  catalyzer,  and  the  colorimetric  comparison 
of  the  solution  with  a  standard  solution  of  permanganate.4 

1  Cf.  H.  S.  Washington,  Prof.  Paper  99,  pp.  17,  20,  21;  Hillebrand  et  al, 
Jour.  Am.  Chem.  Soc.,  28,  p.  233,  1906. 

2Hillehrand,  p.  114. 

8  H.  E.  Walters,  Chem.  News,  84,  p.  239,  1901;  Classen,  1,  p.  487;  Hille- 
brand, pp.  116-118;  Mellor,  pp.  382-384;  Treadwell,  2,  p.  127. 

4  Lead  peroxide,  sodium  bismuthate,  or  potassium  periodate  can  be  used  as 
oxidizers,  instead  of  ammonium  persulphate.  Lead  peroxide  needs  no  silver 
nitrate  as  a  catalyzer,  but  the  solution  must  be  filtered. 


MANGANOUS  OXIDE  221 

There  are  needed  for  this  method  a  standard  solution  of 
manganous  sulphate,  containing  2  milligrams  of  MnO  in  10  c.c., 
and  a  solution  of  silver  nitrate,  both  of  which  have  been  described 
on  pp.  51  and  54. 

About  1  gram  of  the  rock  powder  is  weighed  out  into  a  capa- 
cious platinum  crucible,  or  better,  a  small  platinum  basin,  and 
10  c.c.  of  sulphuric  acid  (1:1)  and  5  c.c.  of  hydrofluoric  acid  are 
poured  in.  After  thorough  mixing  with  a  small  platinum  spatula 
or  wire,  the  mixture  is  heated  gently  until  the  powder  is  wholly 
decomposed,  and  then  more  strongly  until  white  fumes  of  SOs 
are  given  off.  After  cooling,  another  5  c.c.  of  dilute  sulphuric  acid 
are  added  and  the  heating  repeated,  this  being  done  once  or  twice 
more  to  ensure  the  expulsion  of  the  hydrofluoric  acid.  The  last 
time  it  is  best  to  heat  almost  to  dryness.  About  10  c.c.  of  nitric 
acid,  which  should  be  free  from  chlorine,  and  the  same  amount  of 
water  are  added.  The  mixture  is  gently  heated  for  ten  minutes 
or  so,  to  dissolve  all  manganese  that  may  be  present.  The  in- 
soluble residue,  chiefly  calcium  and  possibly  barium  sulphates,  is 
filtered  off  through  a  small  filter  and  washed  with  small  portions 
of  hot  water,  the  filtrate  and  washings  being  caught  in  a  150-c.c. 
beaker.  They  should  not  amount  to  more  than  about  50  c.c. 

To  the  clear  rock  solution  10  c.c.  of  the  silver  nitrate  solu- 
tion are  added  for  each  milligram,  or  0.10  per  cent  of  manganese 
present  in  the  rock.  For  very  silicious  rocks,  as  granites  or 
rhyolites,  10  c.c.  will  be  ample,  but  for  most  rocks  20  or  30  c.c. 
should  be  used.  It  is  essential  to  have  an  excess  of  silver  nitrate, 
and  I,  therefore,  prefer  a  solution  containing  3,  rather  than  2, 
grams  to  the  liter.  If  there  is  a  precipitate  of  silver  chloride,  as 
will  happen  if  the  rock  contains  sodalite  or  scapolite,  the  liquid 
is  to  be  boiled  and  well  stirred  to  coagulate  the  precipitate  and  fil- 
tered into  another  small  beaker,  the  filter  being  washed  twice 
with  a  little  water. 

Ten  c.c.  of  dilute  (1:1)  sulphuric  acid  are  poured  in,  about  2 
grams  of  solid  ammonium  persulphate  are  added,  and  the  beaker 
is  gently  heated  over  a  low  flame.  The  salt  dissolves  with  a 
crackling  noise,  and  as  the  liquid  warms,  the  purple  color  of  the 
permanganate  appears,  which  gradually  deepens  until  it  attains  a 
maximum.  Soon  after  the  color  begins  to  appear,  it  is  well  to 
remove  the  beaker  from  the  flame  and  place  it  in  a  basin  con- 


222  METHODS 

taining  some  cool  water  until  the  color  has  reached  its  greatest 
depth. 

With  some  rocks,  or  under  certain  conditions,  the  color  of  the 
liquid  is  a  peculiar  red,  due  possibly  to  the  presence  of  manganic 
salts.  If  sufficient  silver  nitrate  is  present  this  will  gradually 
change  into  the  true  purple  of  permanganate  solutions.  But  to 
hasten  matters,  the  solution  may  be  decolorized  by  the  cautious 
addition  of  a  few  drops  of  solution  of  sulphur  dioxide,  the  addition 
of  5  or  10  c.c.  of  silver  nitrate  solution  and  another  gram  or  so  of 
persulphate.  On  heating,  the  proper  color  will  now  appear. 

Sometimes,  after  the  proper  coloration  has  begun  at  the  bottom, 
the  rest  of  the  liquid  assumes  a  dirty  brown  color,  due  to  the  sepa- 
ration of  manganese  hydroxide.  This  may  be  caused  by  insuf- 
ficient silver  nitrate,  but  it  is  generally  due  to  insufficiency  of 
sulphuric  acid.  The  liquid  is  cleared  and  decolorized  by  cautious 
addition  of  sulphur  dioxide;  10  c.c.  of  dilute  sulphuric  acid  and 
5  c.c.  of  silver  nitrate  with  about  a  gram  of  persulphate  are  added, 
and  the  solution  brought  to  a  boil.  Sufficient  sulphuric  acid 
should  always  be  added  at  the  beginning  to  prevent  this  annoying 
discoloration,  the  rectification  of  which  will  add  considerably  to 
the  volume  of  the  liquid. 

When  the  proper  color  has  been  obtained  and  the  liquid  has 
cooled  to  room  temperature,  the  solution  is  poured  into  a  100-c.c. 
measuring-flask,  with  stopper,  unless  the  rock  contains  much 
manganese,  when  a  200-  or  250-c.c.  flask  is  to  be  used.  The  solu- 
tion will  retain  its  full  color  unchanged  for  several  days,  the 
rapidity  of  the  change  being  the  less  the  stronger  the  solution,  so 
that  one  should  not  dilute  up  to  the  mark  until  one  is  ready  for 
the  comparison,  if  a  series  of  manganese  determinations  is  to  be 
carried  out.  The  depth  of  color  must,  of  course,  be  considerably 
lighter  than  that  of  the  standard,  so  it  is  well  not  to  dilute  the  test 
solution  until  the  standard  has  been  prepared. 

In  another  100-c.c.  measuring-flask  there  are  placed  by 
means  of  a  10-c.c.  pipette  10,  20,  or  30  c.c.  of  the  standard  man- 
ganese solution,  the  amount  depending  on  the  depth  of  color  of 
the  test  solution.  The  color  of  the  standard,  when  oxidized  to 
permanganate,  must  be  considerably  deeper  than  that  of  the  test 
solution.  Ten  c.c.  is  usually  enough.  The  required  amount  of 
silver  nitrate  solution,  20  c.c.  for  each  10  c.c.  of  manganese  solution, 


MANGANOUS  OXIDE  223 

10  c.c.  of  dilute  sulphuric  acid,  and  2  grams  of  ammonium  per- 
sulphate, are  added.  The  solution  is  boiled  and  oxidized  to  the 
proper  color  exactly  as  with  the  test  solution. 

When  cool  the  standard  is  diluted  to  the  mark,  thoroughly 
mixed,  and  poured  into  a  burette.  Ten  c.c.  are  drawn  off  with  a 
pipette  into  one  of  the  rectangular  glasses  used  in  the  titanium 
determination  (p.  43).  In  the  other  glass  is  placed  some  or  all 
of  the  test  solution.  The  standard  is  now  diluted  with  water 
from  another  burette  until  the  color  of  the  standard  matches 
that  of  the  test  solution.  The  whole  process  is  carried  out  exactly 
as  was  done  with  titanium.  The  calculation  of  the  amount  of 
manganese  is  exactly  analogous  to  that  of  titanium,  due  regard 
being  paid  to  the  amount  of  standard  manganese  solution  used.1 
If  ammonium  persulphate  has  been  added  prior  to  the  precipita- 
tion of  the  main  portion  by  ammonia,  the  weight  of  the  manga- 
nese, calculated  as  Mn304  by  multiplying  the  weight  of  MnO  by 
1.075,  is  to  be  subtracted  from  that  of  the  ignited  ammonia 
precipitate  of  alumina,  etc. 

Gravimetric  Method. — If  the  basic  acetate  method  has  been 
used,  and  it  is  desired  to  determine  manganese  in  the  main  portion, 
it  may  be  done  according  to  the  following  method  described  by 
Hillebrand2  and  used  by  him  in  cases  where  the  colorimetric 
method  is  not  applicable.  For  the  reasons  given  on  page  149 
this  gravimetric  determination  should  not  be  used  unless  the 
analyst  is  forced  to  do  so. 

The  combined  filtrates  from  the  precipitate  of  alumina,  iron, 
etc.,  if  the  basic  acetate  method  has  been  used,  are  evaporated 
down  to  a  bulk  of  about  100  c.c.,  best  in  the  platinum  basin, 
after  ammonia  water  has  been  added  to  alkaline  reaction.  This 
will  in  almost  all  cases  produce  a  precipitate  of  aluminum  and 
ferric  hydroxides,  which  must  be  filtered  off  on  a  small  filter, 
ignited  and  weighed.  It  is  at  this  point  that  there  is  danger  of 
neglecting  to  collect  the  slight  precipitate  of  alumina,  if  the  man- 
ganese is  precipitated  without  previous  filtration,  so  that  any 
alumina  or  iron  present  is  weighed  with  it. 

The  filtrate  is  caught  in  a  200-c.c.  Erlenmeyer  flask,  and  if  the 
platinum  basin  is  stained  brown  by  deposited  manganese,  this  is 

1  For  an  example  see  p.  243. 

2  Hillebrand,  pp.  115-116. 


224  METHODS 

to  be  dissolved  in  a  few  drops  of  hydrochloric  acid  and  a  drop  of 
sulphurous  acid  and  washed  into  the  flask. 

Enough  ammonia  water,  is  added  to  make  the  contents  of  the 
flask  strongly  alkaline,  and  a  current  of  EkS  is  passed  through  it 
for  ten  minutes,  which  precipitates  the  manganese,  and  also  nickel, 
cobalt,  copper  and  zinc,  and  the  platinum  which  may  have  been 
derived  from  the  basin.  The  flask  is  corked  and  allowed  to  stand 
for  twenty-four  hours. 

The  precipitated  sulphides  are  collected  on  a  7-cm.  filter,  and 
washed  with  water  containing  a  little  ammonium  chloride  and 
ammonium  sulphide,  the  flask  being  also  rinsed  out  with  this. 
The  filtrate  is  received  in  a  400-c.c.  beaker,  and  reserved  for  the 
determination  of  lime  and  magnesia  (p.  177). 

The  sulphide  of  manganese  (and  zinc)  is  dissolved  by  passing 
a  few  cubic  centimeters  of  hydrogen-sulphide  water  acidified  with 
one-fifth  of  its  bulk  of  hydrochloric  acid  through  the  filter,  and  the 
filter  is  washed  several  times.  The  liquid  is  received  in  a  small 
porcelain  or  platinum  basin  and  evaporated  to  dryness.  A  few 
drops  of  solution  of  sodium  carbonate  are  added  and  the  contents  of 
the  dish  are  again  evaporated  to  destroy  ammonium  salts,  which 
would  hinder  the  complete  precipitation  of  manganese.  The  dry 
salts  are  then  dissolved  in  about  10  c.c.  of  water  to  which  a  few 
drops  of  hydrochloric  acid  are  added,  and  are  precipitated  with 
sodium  carbonate.  The  manganese  carbonate  is  collected  on  a 
5J-cm.  filter,  washed,  ignited  in  a  weighed  crucible  and  weighed 
as  MnsO4. 

The  black  residue  on  the  filter  may  contain  nickel,  cobalt, 
copper  and  platinum.  The  filter  is  incinerated  in  a  porcelain 
crucible,  and  the  residue  is  dissolved  in  a  few  drops  of  aqua  regia, 
evaporated  to  dryness  in  the  crucible,  dissolved  in  a  little  water  and 
hydrochloric  acid,  and  a  little  strong  hydrogen-sulphide  water  is 
added,  which  will  precipitate  the  copper  and  platinum.  These 
are  filtered  off  on  a  small  filter,  and  in  the  filtrate,  to  which  ammo- 
nia is  added,  nickel  and  cobalt  are  precipitated  by  hydrogen  sul- 
phide. A  few  drops  of  acetic  acid  are  added  and  the  liquid  is 
allowed  to  stand  for  some  hours,  when  the  nickel  (and  cobalt) 
sulphides  are  caught  on  a  5^-cm.  filter,  ignited  and  weighed  as 
oxide.  The  amount  of  cobalt  is  so  small  in  terrestrial  rocks  that 
it  is  not  necessary  to  separate  it  from  the  nickel,  but  its  presence 


TOTAL  SULPHUR,   ZIRCONIA,   BARYTA,  ETC.  225 

may  be  established,  if  desired,  by  testing  the  oxide  with  the  borax 
bead. 

It  is,  however,  almost  always  best  to  determine  nickel  and 
copper  in  a  separate  portion,  as  described  on  p.  238. 


18.  TOTAL  SULPHUR,  ZIRCONIA,  BARYTA,  AND  RARE  EARTHS 

These  constituents  may  be  determined  in  separate  portions, 
but  it  will  be  found  to  be  a  great  economy  of  time  to  determine 
them  in  the  same  portion  by  the  following  plan,  which  was  first 
published  by  Hillebrand,1  and  independently  worked  out  by 
myself.  The  whole  process,  while  apparently  complicated,  in 
reality  takes  very  little  extra  time  for  its  execution,  as  the  volumes 
of  liquid  are  small,  and  the  various  operations  may  be  carried  out 
during  pauses  between  the  main  determinations,  when  solutions 
are  being  evaporated,  etc. 

Decomposition. — For  this  set  of  determinations  1  gram  of 
rock  powder  is  sufficient.2  About  this  amount  is  weighed  out  into 
a  weighed  platinum  crucible,  mixed  with  four  or  five  times  its 
weight  of  sodium  carbonate,  and  the  mixture  is  fused  precisely 
as  has  been  described  above  (p.  131). 

If  pyrite  is  present,  and  it  is  desired  to  determine  the  sulphur, 
a  small  quantity,  about  \  gram,  of  powdered  potassium  nitrate 
is  mixed  with  the  carbonates,  which  should  have  been  tested  to  see 
if  they  are  free  from  sulphur  or  sulphates.  If  much  niter  is  used 
the  crucible  is  liable  to  be  attacked.  The  reaction  between  the 
nitrate  and  the  carbonates  gives  rise  to  considerable  effervescence, 
and  the  fusion  should,  therefore,  be  carried  on  cautiously,  and  at 
as  low  a  temperature  as  possible,  till  all  nitrous  fumes  have  dis- 
appeared. The  temperature  may  then  be  raised  and  the  operation 
carried  on  as  above. 

When  the  cake  is  perfectly  cold  it  is  detached  from  the  crucible 
and  thoroughly  leached  with  water,  till  all  soluble  matter  is 
dissolved,  a  drop  or  two  of  alcohol  being  added  to  reduce  any 
sodium  manganate  which  may  be  present. 

1  Hillebrand,  p.  138. 

2  If  the  rare  earths  are  to  be  looked  for,  2  grams  should  be  taken.     In  this 
case  8  grams  of  sodium  carbonate  are  used  for  the  fusion. 


226  METHODS 

Of  the  constituents  which  immediately  concern  us,  the  sul- 
phur (that  as  sulphide  as  well  as  that  as  sulphate)  passes  into 
solution  as  sodium  sulphate,  while  the  baryta  and  zirconia  remain 
undissolved,  the  former  as  barium  carbonate  and  the  latter  as 
sodium  zirconate.  The  rare  earths  are  also  insoluble. 

Sulphur. — The  liquid  is  filtered  through  a  7-cm.  filter,  as 
little  as  possible  of  the  undissolved  residue  being  brought  on 
this,  and  the  residue  and  filter  are  well  washed  with  a  very  dilute 
solution  of  sodium  carbonate  to  prevent  turbid  washings  (Hille- 
brand) .  The  further  treatment  of  the  residue  will  be  found  below 
under  Zirconia. 

If  the  filtrate  is  yellow  the  presence  of  chromium  is  indicated 
and  this  may  be  determined  either  in  this  liquid,  or  in  a  separate 
portion,  by  the  colorimetric  method,  as  described  on  p.  237.  In 
most  rocks  the  filtrate  is  colorless,  when  the  absence  of  chromium 
may  be  noted  in  the  tabulation  of  the  analysis,  and  this  constit- 
uent need  not  be  looked  for.  Assuming  that  it  is  absent,  or  even 
when  it  is  present,1  we  may  proceed  to  the  determination  of  the 
sulphur  in  this  liquid. 

The  filtrate,  which  should  amount  to  from  150-250  c.c.  in 
a  400-c.c.  beaker,  is  colored  with  two  or  three  drops  of  methyl- 
orange  solution,  and  hydrochloric  acid  is  added  gradually  through 
a  small  funnel  with  curved  tip,  the  beaker  being  covered  to  pre- 
vent loss,  till  the  original  orange  color  changes  to  pink,  indicating 
that  the  liquid  is  acid.  About  half  or,  at  most,  1  gram  of  barium 
chloride  dissolved  in  25  c.c.  of  water  is  added  to  the  boiling  liquid, 
the  cover  and  sides  washed  down,  and  the  beaker  allowed  to  stand 
till  the  barium  sulphate  has  settled.  There  is  little  danger  of 
silica  contaminating  the  barium  sulphate,  in  the  bulk  of  liquid 
recommended  above,  but  if  this  should  happen  it  is  removed  later. 
It  is  obvious  that  failure  of  barium  chloride  to  produce  a  precipitate 
indicates  the  absence  not  only  of  S,  but  of  SOs.  In  this  case  this 
last  need  not  be  looked  for,  but  both  may  be  stated  in  the  tabulation 
as  absent. 

The  liquid  is  filtered,  all  the  barium  sulphate  being  brought 
on  a  small  filter  (7  cm.)  by  means  of  a  rubber-tipped  rod  and  the 

1  The  free  hydrochloric  acid  present  prevents  the  precipitation  of  barium 
chromate.  The  colorimetric  determination  of  chromium,  if  it  is  done  here, 
should  precede  the  determination  of  sulphur. 


•TOTAL  SULPHUR,   ZIRCONIA  ,  BARYTA,  ETC.  227 

wash-bottle;  the  filter  is  now  well  washed.  The  filter  is  ignited  in 
a  small  weighed  crucible,  the  barium  sulphate  evaporated  with 
a  few  drops  of  hydrofluoric  and  one  of  sulphuric  acids  to  expel  any 
silica  possibly  present,  again  ignited  and  weighed.  Further 
purification  of  the  barium  sulphate  is  seldom  necessary. 

For  the  very  small  amounts  of  sulphur  present  in  most  igneous 
rocks  the  precautions  and  refinements  that  have  been  suggested  l 
in  the  precipitation  of  barium  sulphate,  to  insure  its  purity,  are 
not  necessary. 

If  no  sulphur  trioxide  is  present  in  the  rock,  the  weight  of 
barium  sulphate  is  multiplied  by  0.137  to  reduce  it  to  S.  If 
sulphur  trioxide  is  present,  the  weight  of  barium  sulphate  is  mul- 
tiplied by  0.343  to  reduce  it  to  SOs,  which  is  changed  to  percentage 
figures  by  division  by  the  weight  of  substance  taken.  From  this 
the  percentage  of  sulphur  trioxide  present  in  the  rock  as  obtained 
in  a  separate  portion  (p.  231)  is  deducted,  and  the  remainder 
multiplied  by  0.401  to  reduce  the  SOs  to  S. 

Zirconia. — The  whole  of  this  is  present  as  sodium  zirconate 
in  the  residue  insoluble  in  water.  The  small  part  of  this  which 
adheres  to  the  filter  is  washed  back  into  the  beaker  containing 
the  bulk  of  the  residue,  by  holding  the  funnel  sidewise  and  directing 
a  strong  stream  of  water  from  the  wash-bottle  against  all  parts 
of  the  filter,  the  liquid  dropping  into  the  beaker  beneath.  With 
care,  and  if  done  while  the  residue  is  still  moist,  the  removal  can 
easily  be  made  complete.  It  is  of  no  consequence  if  a  little  remains 
on  the  filter. 

To  the  contents  of  the  beaker,  the  bulk  of  which  should  be 
less  than  50  c.c.,  not  more  than  three  or  four  drops  of  concentrated 
sulphuric  acid  are  added.  A  larger  amount  is  to  be  avoided,  as 
too  much  free  sulphuric  acid  prevents  the  entire  precipitation  of 
the  zirconia  (Hillebrand),  and  also  retards  filtration  through 
action  on  the  filter-paper.  7£he  liquid  is  warmed  (not  boiled)  till 
all  effervescence  ceases,  and  another  drop  or  two  of  sulphuric 
acid  is  added  to  see  if  solution  has  been  complete.  The  liquid 
should  be  distinctly  acid.  The  liquid  is  filtered  through  the  original 
filter  into  a  flask  of  about  100  c.c.,  and  the  filter  and  beaker 
are  washed  several  times  with  small  quantities  of  warm  water. 

1  Allen  and  Johnston,  Jour.  Am.  Chem.  Soc.,  32,  p.  588,  1910;  Johnston 
and  Adams,  op.  cit.,  33,  p.  830,  1911. 


228  METHODS 

The  filtrate  now  contains  all  the  zirconia  as  sulphate,  while 
the  baryta  remains  behind  as  insoluble  barium  sulphate,  along 
with  strontia  and  some  lime  and  silica.  For  the  treatment  of 
this  insoluble  portion,  see  p.  229. 

To  the  filtrate  in  the  flask  is  now  added  about  5  c.c.  of  hydrogen 
peroxide,  or  enough  to  cause  a  permanent  yellow  coloration, 
and  then  1  c.c.  of  a  solution  of  a  soluble  phosphate,  such  as  micro- 
cosmic  salt.  The  flask  is  filled  nearly  to  the  neck,  if  not  so  already 
and  set  aside  in  a  cool  place  for  at  least  twenty-four  hours,  and 
preferably  for  twice  that  length  of  time.  If  the  yellow  color  dis- 
appears, a  little  more  hydrogen  peroxide  is  to  be  added. 

The  zirconia  separates  as  a  flocculent  precipitate  of  basic  zirco- 
nium phosphate,  which  may  easily  be  overlooked  unless  the  flask  is 
gently  agitated.  It  is  almost  orentirely  free  from  titanium  the  precip- 
itation of  which  is  prevented  by  the  hydrogen  peroxide.  However 
small  the  precipitate  may  appear  it  is  filtered  off  through  a  5^-cm. 
filter,  and  well  washed.  The  filtrate  is  reserved  for  the  determina- 
tion of  the  rare  earths,  if  these  are  to  be  looked  for  (p.  229). 

For  most  rocks,  in  which  the  amount  of  ZrCb  is  very  small, 
further  purification  is  unnecessary,  and  the  filter  and  precipitate 
are  ignited  in  a  small  weighed  crucibile,  and  weighed  as  basic 
zirconium  phosphate.  This  contains  51.8  per  cent  of  ZrO2, 
but  for  the  minute  quantities  usually  present  it  will  suffice  to 
multiply  it  by  0.5  to  reduce  it  to  ZrC>2.  The  percentage  amount 
of  ZrO2  is  to  be  subtracted  from  that  of  the  ignited  precipitate  by 
ammonia  to  arrive  at  the  correct  figure  for  alumina. 

If  the  precipitate  is  large,  or  if  extreme  accuracy  is  desired, 
the  purification  recommended  in  every  case  by  Hillebrand  may 
be  carried  out.  The  ignited  precipitate  (unweighed)  is  fused 
with  a  very  little  sodium  carbonate,  leached  with  water  and 
filtered.  The  small  filter  and  contents  are  ignited  and  then 
fused  with  a  small  lump  of  acid  potassium  sulphate.  The 
cooled  melt  is  dissolved  in  hot  water  and  a  drop  or  two  of  dilute 
sulphuric  acid.  To  the  solution  in  the  crucible  a  little  hydrogen 
peroxide  and  a  few  drops  of  soluble  phosphate  are  added,  and  the 
covered  crucible  is  set  aside  as  before.  The  precipitated  zirconium 
phosphate  now  free  from  titanium,  is  collected,  ignited  and 
weighed  as  above.  For  identification  of  the  zirconia  the  reader 
is  referred  to  Hillebrand. 


TOTAL  SULPHUR,   ZIRCONIA,   BARYTA,  ETC.  229 

Baryta. — The  residue  left  on  solution  of  the  zirconia  in  dilute 
sulphuric  acid  contains  all  the  baryta,  with  traces  of  strontia  and 
often  much  lime,  as  insoluble  sulphates.  To  bring  these  into 
solution  it  is  collected  on  a  small  filter,  as  described  above,  the 
filter  and  contents  are  ignited  in  a  small  crucible  and  fused  with 
about  1  gram  of  sodium  carbonate,  the  fusion  being  continued 
for  ten  to  fifteen  minutes  to  permit  the  complete  conversion  of 
the  barium  sulphate  into  carbonate.  There  should  be  enough 
carbonate  for  this  conversion  by  mass  action. 

The  cake  is  leached  with  warm  water,  which  may  be  done  in 
the  crucible,  filtered  through  a  small  filter,  and  well  washed. 
After  a  fresh  250-c.c.  beaker  has  been  placed  beneath  the  funnel, 
the  carbonates  are  dissolved  on  the  filter  in  a  very  little,  warm, 
dilute  hydrochloric  acid,  and  the  filter  is  well  washed.  The  liquid 
in  the  beaker  is  made  up  to  at  least  150  c.c.  to  prevent  precipi- 
tation of  strontium  and  calcium  sulphates,  and  2  or  3  c.c.  of  con- 
centrated sulphuric  acid  are  added.  After  standing  for  twenty- 
four  hours,  the  precipitated  barium  sulphate  is  filtered  off,  ignited, 
and  weighed.  It  will  seldom  be  necessary  to  purify  it  for  con- 
tamination by  calcium  or  strontium.  Multiplication  of  the  weight 
of  BaSO4  by  0.66  reduces  it  to  BaO. 

Rare  Earths. — It  may  be  desired  to  determine  the  so-called 
rare  earths,  oxides  of  the  metals  of  the  cerium  and  yttrium  groups, 
as  appreciable  amounts  of  these  may  be  present  in  highly  sodic 
rocks  (p.  18).  This  may  be  done  by  a  simple  method  devised 
by  Hillebrand,1  which  is  quite  accurate  enough  for  the  very 
small  amounts  that  are  ever  found  in  rocks.  If  the  total  percent- 
age of  rare  earths  amounts  to  several  tenths  of  a  per  cent  it  may  be 
well  to  separate  them  into  the  earths  of  the  cerium  and  yttrium 
groups. 

For  the  determination  of  the  rare  earths  it  is  most  convenient 
to  use  the  acid  filtrate  from  the  precipitate  of  zirconium  phos- 
phate, inasmuch  as  zirconium  should  always  be  determined  if  the 
composition  of  the  rock  warrants  the  search  for  the  rare  earths, 
because  both  are  found  together  in  relatively  largest  (though 
usually  absolutely  small)  amounts  in  highly  sodic  rocks.2 

1  Hillebrand,  p.  143;  M.  Dittrich,  Berichte,  41,  p.  4373,  1908;  J.  Moroze- 
wicz,  Bull.  Ac.  Sci.,  Crac.,  1909,  p.  207. 

2  Cf .  H.  S.  Washington,  Trans.  Am.  Inst.  Min.  Eng.,  1908,  p.  755. 


230  METHODS 

The  filtrate  from  the  zirconium  phosphate  is  precipitated  with  a 
solution  of  potassium  or  sodium  hydroxide  in  decided  but  not 
too  great  an  excess.  This  will  keep  the  alumina  in  solution 
but  will  precipitate  the  rare  earths  along  with  the  ferric  oxide  and 
titanium  dioxide.  The  precipitate  is  washed  several  times  with 
hot  water,  brought  on  a  9-  or  1 1-cm.  filter  where  it  is  again  washed 
twice  or  thrice. 

It  is  then  washed  by  a  jet  from  the  wash-bottle  into  a  small 
platinum  basin,  a  few  cubic  centimeters  of  hydrofluoric  acid  are 
added,  enough  to  just  dissolve  the  precipitate,  and  the  liquid  is 
evaporated  to  dry  ness.  No  sulphuric  acid  is  added.  The  soluble 
fluorides,  of  iron  and  possibly  titanium,  are  dissolved  in  a  little 
water  containing  a  few  drops  of  hydrofluoric  acid,  and  the  insoluble 
rare  earth  fluorides  are  collected  in  a  5^ -cm.  filter,  supported  in  a 
small  rubber  funnel.  In  these  operations  only  platinum  vessels 
can  be  used,  as  glass  or  porcelain  would  be  attacked  by  the  hydro- 
fluoric acid. 

The  washed  filter  and  contents  are  ignited  in  a  platinum  cru- 
cible, a  few  cubic  centimeters  of  dilute  sulphuric  acid  added,  and 
the  contents  are  evaporated  to  dry  ness.  The  small  mass  is  taken 
up  with  a  little  dilute  hydrochloric  acid,  transferred  to  a  small 
beaker  and  the  rare  earths  are  precipitated  as  hydroxides  with  a 
slight  excess  of  ammonia  water.  The  hydroxides  are  slightly 
washed  and  then  dissolved  on  the  filter  in  a  little  dilute  hydro- 
chloric acid,  and  precipitated  with  a  decided  excess  of  concen- 
trated solution  of  ammonium  oxalate.  The  oxalate  is  collected 
on  a  5J-cm.  filter,  washed  with  water  containing  a  little  of  this 
salt,  ignited  and  weighed  in  a  platinum  crucible,  as  (Ce,Y)20s. 

If  the  weight  is  more  than  a  few  milligrams  the  earths  of  the 
two  main  groups  may  be  separated  by  dissolving  the  oxides  in  a 
very  small  volume  of  dilute  hydrochloric  acid,  evaporating  down 
almost  to  crystallization,  adding  a  few  drops  of  water,  and,  finally, 
a  few  cubic  centimeters  of  cold  concentrated  solution  of  normal 
potassium  sulphate.  This  should  have  been  prepared  before- 
hand, and  must  contain  some  undissolved  potassium  sulphate 
crystals.  On  standing,  a  double  sulphate  of  potassium  and 
cerium  separates,  as  this  is  almost  insoluble  in  the  potassium 
sulphate  solution,  whereas  the  double  sulphates  of  the  metals  of 
the  yttrium  group  are  easily  soluble  in  it.  The  precipitate  is  fil- 


SULPHUR  TRIOXIDE  231 

tered  off  in  a  small  filter,  washed  with  the  concentrated  potassium 
sulphate  solution,  dissolved  in  dilute  hydrochloric  acid,  precip- 
itated with  ammonia  water,  and  the  precipitate  is  ignited  and 
weighed.  The  ignited  oxide  is  the  dioxide  of  cerium,  CeCb,  of  a 
peculiar  dark,  reddish  brown  color.  This  may  be  calculated  into 
the  sesquioxide,  Ce2O3,  in 'which  form  the  rare  earths  are  stated  in 
the  analysis,  by  multiplying  the  weight  by  the  factor  0.95,  but  the 
weight  is  usually  so  small  as  to  make  this  an  ultra-refinement. 

The  earths  of  the  yttrium  group  may  be  determined  by  differ- 
ence from  the  weight  of  the  mixed  oxides,  or  may  be  precipitated 
as  hydroxides,  ignited  and  weighed  as  ¥263.  Further  separation  of 
the  elements  of  the  two  groups  is  neither  called  for  nor  usually 
practicable.  The  weight  of  the  rare  earths  is  to  be  subtracted 
from  that  of  the  alumina  precipitate. 


19.  SULPHUR  TRIOXIDE  ] 

Sulphur  trioxide,  which  is  present  usually  in  hauyne  and  nose- 
lite,  both  soluble  in  hydrochloric  acid,  is  determined  in  a  separate 
portion.  About  1  gram  is  weighed  out  (p.  129)  into  a  250-c.c. 
beaker,  and  gently  boiled  with  50  c.c.  of  dilute  hydrochloric  acid 
(1:5).  If  pyrite  or  pyrrhotite  are  present,  a  stream  of  carbon 
dioxide  should  enter  by  the  lip  of  the  covered  beaker,  and  fill  the 
space  beneath  the  cover  before  boiling  is  begun.  It  is,  of  course, 
continued  during  the  boiling.  In  this  way  any  pyrite  remains 
unattacked,  while  seven-eighths  of  the  sulphur  of  pyrrhotite  goes 
off  as  hydrogen  sulphide,  the  remaining  one-eighth  being  precip- 
itated as  sulphur.  This  need  not  be  filtered  off,  as  it  is  burned 
in  the  subsequent  ignition. 

After  boiling  for  about  a  quarter  of  an  hour,  the  liquid  is  fil- 
tered through  a  9-cm.  filter,  and  the  residue  and  filter  are  washed. 
The  volume  of  liquid  should  be  about  200  c.c.,  to  prevent  pre- 
cipitation of  silica.  It  is  then  precipitated,  best  while  hot,  with 
an  excess  of  barium-chloride  solution,  allowed  to  stand  for  some 
time  and  the  barium  sulphate  is  filtered  off,  well  washed,  ignited 
and  weighed.  To  guard  against  contamination  by  silica  it  is 
always  as  well  to  evaporate  the  ignited  precipitate  with  a  few 

1  Hillebrand,  p.  199. 


232  METHODS 

drops  of  hydrofluoric  and  sulphuric  acids,  and  again  ignite.    The 
weight  of  BaSO4  is  multiplied  by  0.343  to  obtain  that  of  SO3. 

Before  determining  sulphur  or  sulphuric  anhydride,  the  con- 
dition in  which  the  sulphur  exists  in  the  rock  should  be  investi- 
gated. The  microscope  will  usually  reveal  the  presence  of  pyrite 
or  pyrrhotite,  as  well  as  noselite  or  haiiyne.  If  not,  the  rock 
powder  should  be  boiled  with  a  little  dilute  hydrochloric  acid,  and 
if  hydrogen  sulphide  is  evolved  the  presence  of  pyrrhotite  may  be 
inferred,  as  the  lazurite  molecule  is  not  apt  to  be  found  in  rocks. 
A  little  of  the  filtered  liquid  may  be  tested  with  barium  chloride 
for  S03. 

20.  CHLORINE  1 

While  Hillebrand  recommends  fusion  with  chlorine-free  sodium 
carbonate,  to  ensure  getting  all  the  chlorine,  yet  it  is  not  only 
difficult  to  procure  such  a  reagent,  but  the  operation  will  be 
somewhat  long  and  complex.  For  nearly  all  purposes  simple 
solution  in  nitric  acid,  if  desired  with  the  addition  of  some  hydro- 
fluoric acid,  will  be  quite  sufficient. 

About  1  gram  of  rock  powder  is  weighed  out  into  a  250-c.c. 
beaker  and  boiled  with  50  c.c.  of  dilute  nitric  acid  (1:5)  which 
should  have  been  previously  tested  as  to  freedom  from  chlorine; 
or  a  blank  determination  is  to  be  made  with  the  same  volume  of 
the  acid  to  allow  of  a  suitable  correction  if  chlorine-free  acid  is 
unobtainable.  If  the  addition  of  hydrofluoric  acid  is  desired  the 
digestion  should  be  carried  out  in  a  capacious  crucible  or  small 
platinum  basin. 

After  heating  gently  on  the  water  bath  for  a  quarter  of  an 
hour,  the  liquid  is  filtered,2  the  filter  and  residue  are  well  washed 
and  the  filtrate  is  precipitated  with  excess  of  silver  nitrate  solu- 
tion. It  is  heated  with  constant  stirring,  to  coagulate  the  silver 
chloride.  If  the  precipitate  is  at  all  considerable,  it  is  filtered 
through  a  small  filter  and,  after  washing,  is  dissolved  on  the 
filter  with  ammonia  water,  reprecipitated  by  acidifying  with 
nitric  acid  to  free  it  from  possibly  contaminating  silica,  and 

1  Hillebrand,  p.  183. 

2  A  rubber  or  platinum  funnel  and  the  platinum  basin  are  to  be  used  if 
hydrofluoric  acid  has  been  added. 


FLUORINE  233 

collected  in  a  weighed  Gooch  crucible.  After  washing,  it  is  dried, 
heated  to  incipient  fusion  and  weighed.  The  weight  of  the 
AgCl  multiplied  by  0.247,  or  0.25  for  small  amounts,  will  give 
the  weight  of  chlorine  present. 

If  the  precipitate  is  very  small,  Hillebrand  recommends 
that  it  be  collected  on  a  small  paper  filter,  which  is  then  dried, 
wound  up  in  a  weighed  platinum  wire  and  carefully  ignited. 
The  increased  weight  of  the  wire  is  due  to  the  metallic  silver  of 
the  chloride  which  has  alloyed  with  the  platinum,  and  is  multi- 
plied by  0.33  to  arrive  at  the  chlorine. 

If  the  chlorine  is  present  only  in  minerals  of  the  sodalite  group, 
solution  in  nitric  acid  alone  will  usually  be  sufficient.  But  if 
scapolites  are  present,  some  of  which  are  not  attacked  by  this 
acid,  the  addition  of  hydrofluoric  acid  will  be  necessary. 

In  the  determination  of  chlorine  the  reagents  used  must  be 
free  from  chlorine,  and  a  duplicate  operation  in  blank  with  the 
same  quantities  will  be  a  wise  precaution.  Rock  specimens  col- 
lected near  the  seashore  are  sometimes  contaminated  with  sodium 
chloride  derived  from  sea-water.  This  may  be  estimated  in  a 
separate  portion  by  thorough  washing  on  a  filter  with  warm  water, 
and  determination  of  the  chlorine  dissolved  out.  This  is,  of 
course,  to  be  deducted  from  the  amount  of  chlorine  which  is  found 
by  the  previous  method,  and  its  equivalent  amount  of  Na2O 
from  that  of  this  constituent  already  found. 

21.  FLUORINE  1 

It  is  only  in  very  exceptional  rocks  that  fluorine  occurs  in 
more  than  one  or  two-tenths  of  1  per  cent.  As  it  is  usually  present 
only  as  a  constituent  of  apatite,  its  amount  may  be  calculated 
from  that  of  the  phosphorus  pentoxide  present  with  sufficient 
accuracy  for  most  purposes.  The  methods  for  its  determination 
are  somewhat  tedious,  and,  as  it  is  of  apparently  small  import  in 
theoretical  petrology,  the  determination  of  fluorine  may  generally 
be  dispensed  with. 

It  majr  be  determined  either  gravi metrically  or  colorimetrically. 
Of  the  former  the  old  method  of  Berzelius  may  be  used,  with 
modifications  proposed  by  Penfield  and  Minor.2 

1  Hillebrand,  pp.  184-193;  Mellor,  pp.  637-640. 

2  Penfield  and  Minor,  Am.  Jour.  Sci.,  47,  p.  388,  1894. 


234  METHODS 

About  2  grams  of  the  rock  powder  are  fused  with  five  times 
its  weight  of  alkali  carbonates,  and  the  cake  is  thoroughly  leached 
with  hot  water,  filtered  and  washed.  The  filtrate  contains  all 
the  fluorine  as  alkali  fluorides.  While  still  hot  about  5  grams 
of  powdered  ammonium  carbonate  are  added  to  the  filtrate,  and 
when  cold  about  the  same  amount  is  again  added.  The  beaker  is 
allowed  to  stand  for  about  twelve  hours,  the  precipitate  is  filtered 
off  and  washed,  and  the  ammonium  carbonate  in  the  filtrate  is 
decomposed  by  heating  on  the  water-bath  till  no  more  carbon 
dioxide  is  given  off.  About  5  c.o.  of  a  solution  of  zinc  oxide  in 
strong  ammonia  water  is  added  and  the  liquid  is  evaporated  till 
there  is  no  more  odor  of  ammonia.  After  filtering  off  the  precip- 
itate and  washing,  nitric  acid  is  added  to  the  filtrate  till  the  alkali 
carbonate  is  nearly,  but  not  entirely,  decomposed.  If  too  much 
is  added,  a  solution  of  sodium  carbonate  is  poured  in  to  a  decided 
alkaline  reaction. 

As  chromic  and  phosphoric  acids  may  be  present,  Hillebrand 
recommends  the  addition  at  this  point  of  silver  nitrate  in  excess, 
which  will  precipitate  these  substances.  The  liquid  is  heated  and 
filtered,  the  excess  of  silver  precipitated  by  sodium  chloride, 
again  heated  to  coagulation  and  again  filtered,  when  a  little 
sodium  carbonate  is  added  to  alkaline  reaction.  If  no  chromium 
or  phosphorus  is  present,  or  only  small  amounts,  the  addition 
of  silver  nitrate  may  be  dispensed  with. 

The  heated  filtrate,  which  contains  alkali  carbonate  and 
fluoride,  and  which  must  not  contain  ammonium  salts,  is  now 
precipitated  with  an  excess  of  calcium  chloride.  The  precipitate 
of  calcium  carbonate  and  fluoride  is  collected  on  a  filter,  placed  in 
a  weighed  platinum  crucible,  dried  and  ignited  gently.  A  little 
water  and  1  or  2  c.c.  of  acetic  acid  are  poured  in,  and  the  covered 
crucible  is  heated  for  some  time  on  the  water-bath,  and  finally  the 
excess  of  acid  is  evaporated  with  the  cover  off  of  the  crucible. 

Hot  water  is  poured  on  the  dry  salts,  and  the  contents  of  the 
crucible  are  filtered  through  a  small  filter  and  washed.  The  filter 
with  its  contents  are  again  ignited  in  the  same  crucible,  and  the 
digestion  with  dilute  acetic  acid  and  evaporation  are  gone  through 
with  again.  The  ignition  of  the  filters  and  the  digestion  with 
dilute  acetic  acid  are  repeated  till  all  the  calcium  carbonate  and 
oxide  are  dissolved  as  acetate,  as  is  shown  by  the  absence  of  a 


CARBON  DIOXIDE  235 

residue  on  evaporation  of  a  few  drops  of  the  filtrate  on  platinum 
foil.1 

The  filter  and  purified  calcium  fluoride  are  finally  gently 
ignited  in  the  crucible  and  weighed.  Multiplication  of  the 
weight  of  CaF2  by  0.49,  or  division  by  2  in  most  cases,  gives  the 
amount  of  fluorine.  For  possible  corrections  see  Hillebrand. 

A  colorimetric  method  of  determining  the  small  amounts  of 
fluorine  found  in  rocks,  based  on  the  bleaching  effect  of  fluorine 
on  hydrogen  peroxide  solutions  of  titanium,  has  been  proposed 
by  G.  Steiger.2  The  method  in  brief  consists  in  fusing  the  rock 
with  sodium  carbonate,  leaching  with  water,  and  mixing  a  definite 
amount  of  standard  titanium  solution  with  the  filtrate,  which  is 
compared  colorimetrically  with  a  fluorine-free  titanium  solution. 
The  percentage  of  fluorine  is  shown  by  reference  to  a  curve  which 
has  been  determined  empirically,  and  for  this  and  other  details 
the  student  is  referred  to  the  original  paper  or  to  Hillebrand. 

Steiger's  method  has  been  modified  by  Merwin,3  who  also 
gives  the  curves  determined  from  his  empirical  data.  For  this 
method  the  student  would  best  consult  Merwin's  paper,  the  por- 
tion of  the  paper  relating  to  the  fluorine  determination  is  also 
given  by  Hillebrand  (pp.  191-193). 

These  methods,  which  are  only  adapted  to  small  amounts  of 
fluorine,  may  be  recommended  for  use  in  rock  analysis  for  their 
rapidity,  simplicity,  and  greater  accuracy  than  the  usual  gravi- 
metric methods.  It  may,  however,  be  repeated  that  it  is  seldom 
worth  while  to  determine  fluorine  in  rocks. 

22.  CARBON  DIOXIDE  4 

As  all  the  minerals  which  contain  this  constituent  are  soluble 
in  hydrochloric  or  nitric  acid  with  evolution  of  carbon  dioxide 
(dolomite  and  siderite  only  on  warming),  its  qualitative  presence 
may  be  easily  established  by  warming  the  rock  powder  with  a 

1  Penfield  and  Minor  show  that  the  addition  of  acetic  acid  in  large  amount 
at  a  time  leads  to  loss  of  calcium  fluoride. 

2  G.  Steiger,  Jour.  Am.  Chem.  Soc.,  30,  p.  219,  1908;  Hillebrand,  pp.  188- 
193. 

3  H.  E.  Merwin,  Am.  Jour.  Sci.,  28,  p.  124,  1909. 

4  Classen,  2,  pp.  653-656;    Fresenius,  1,  pp.  493-495;    Hillebrand,  pp. 
179-181;  Mellor,  pp.  553-555;  Treadwell,  2,  pp.  380-382. 


236  METHODS 

little,  somewhat  dilute  nitric  or  hydrochloric  acid,  and  noting 
whether  effervescence  ensues.  This  is  done  in  a  test-tube.  If 
there  is  no  marked  effervescence,  one  may  make  sure  of  the 
reaction,  as  Hillebrand  suggests,  by  inclining  the  tube  and  exam- 
ining the  upper  side  with  a  lens  for  the  stream  of  minute  bubbles. 
Before  the  addition  of  the  acid  the  powder  should  be  well  stirred 
up  with  warm  water  to  drive  out  any  mechanically  attached  air, 
bubbles  of  which  might  be  mistaken  for  CO2.  If  the  rock  contains 
considerable  pyrrhotite,  the  evolution  of  hydrogen  sulphide  may 
be  mistaken  for  that  of  carbon  dioxide,  but  the  former  is  easily 
recognizable  by  its  characteristic  odor,  as  well  as  by  the  blacken- 
ing of  paper  soaked  in  lead  acetate  solution  to  which  a  drop  of 
ammonia  has  been  added. 

The  determination  of  carbon  dioxide  is  effected  by  the  usual 
method,  which  is  so  well-known  that  a  brief  description  will 
suffice.  Any  of  the  well-known  forms  of  apparatus  may  be 
used,  and  if  many  determinations  are  to  be  made  it  will  be  as 
well  to  have  one  permanently  set  up. 

At  least  2  or  3  grams  of  rock  powder  are  weighed  out  into  a 
small  flask.  After  mixing  the  powder  with  some  water,  the  flask 
is  connected  on  one  side  with  a  cylinder  filled  with  soda-lime 
or  sticks  of  caustic  alkali,  to  free  the  air  from  CO2.  On  the  other 
side  it  is  connected  with  an  upward  inclined  condenser,  then  a 
U-tube  containing  glass  beads  wet  with  concentrated  sulphuric 
acid,  which  may  be  designated  as  a.  Tube  a  is  followed  by  U- 
tubes  6,  c,  d,  e,  /,  and  g  in  the  order  given.  Tube  6  contains 
granular,  anhydrous  calcium  chloride;  c  contains  pieces  of 
pumice  which  have  been  soaked  in  a  copper  sulphate  solution  and 
heated  at  150°  for  several  hours.  At  this  temperature  the  cal- 
cium sulphate  is  partly  dehydrated  and  easily  retains  both  the 
H2S  and  HC1  which  may  escape  from  the  flask.  Tube  d  con- 
tains granular,  anhydrous  calcium  chloride;  e  contains  soda-lime 
and  /  contains  soda-lime  in  the  arm  nearest  e  and  granular  an- 
hydrous calcium  chloride  in  the  other  arm.  Tubes  e  and  /  are 
the  absorption  tubes,  both  of  which  must  be  weighed  carefully. 
Tube  g  contains  calcinm  chloride  in  the  arm  nearest  /  and  soda- 
lime  in  the  other  arm. 

After  weighing  the  soda-lime  U-tubes  and  connecting  them  in 
place,  the  whole  apparatus  is  filled  with  dry  and  carbon-dioxide- 


CHROMIUM  AND  VANADIUM  237 

free  air  by  means  of  an  aspirator  attached  to  the  last  U-tube. 
About  10  c.c.  of  dilute  hydrochloric  acid  are  added  to  the  flask 
containing  the  powder  and  its  contents  boiled  gently  while  a  slow 
current  of  air  free  from  carbon  dioxide  is  passing.  In  ten  or 
fifteen  minutes  decomposition  is  complete.  The  flame  is  now 
removed  and  the  current  of  air  continued  for  some  time  longer 
in  order  to  drive  all  CCb  into  the  absorption  tubes. 

The  U-tubes  are  then  removed,  carefully  closed,  and  allowed 
to  cool  thoroughly,  as  the  absorption  of  the  carbon  dioxide  by  the 
soda-lime  gives  rise  to  considerable  heat.  They  are  then  weighed, 
the  increase  being  the  amount  of  C02  in  the  portion  of  rock 
powder  taken. 

A  very  simple  and  sufficiently  accurate  apparatus  for  the 
indirect  determination  of  carbon  dioxide  by  loss  of  weight  has 
been  described  by  Kreider.1 

23.  CHROMIUM  AND  VANADIUM  2 

These  constituents  are  so  seldom  present  in  appreciable 
amount  in  silicate  igneous  rocks  that  the  analyst  will  not  often 
be  called  on  to  determine  them. 

Chromium  is  occasionally  to  be  determined  in  such  rocks  as 
dunites,  peridotites,  pyroxenites,  etc.,  and  for  this  the  colori- 
metric  method  recommended  by  Hillebrand  is  to  be  used.  It 
is  briefly  summarized  here. 

If  the  filtrate  from  the  leached  sodium  carbonate  melt  for  sul- 
phur, baryta,  and  zirconia,3  is  yellow,  this  may  be  used  for  the 
chromium  determination.  It  is  evaporated  down  to  a  bulk  of  less 
than  100  c.c.,  and  the  chromium  is  determined  as  described  below. 
The  total  sulphur  may  be  determined  afterwards  in  this  solution 
by  precipitation  as  barium  sulphate  (p.  225). 

If  it  is  desired  to  determine  chromium  in  a  separate  portion,  at 
least  2  grams  of  the  rock  powder  are  thoroughly  fused  with  four 
times  its  weight  of  sodium  carbonate,  and  the  cake  is  extracted 
with  water,  as  in  the  method  for  total  sulphur  (p.  225).  A  few 
drops  of  either  ethyl  or  methyl  alcohol  are  added  to  destroy  the 

1  Am.  Jour.  Sci.,  19,  p.  188,  1905;  cf.  Hillebrand,  p.  181. 

2  Hillebrand,  pp.  147-150;   Mellor,  pp.  472-474. 

3  See  p.  225. 


238  METHODS 

color  of  sodium  manganate,  and  the  liquid  is  filtered.  If  the 
yellow  color  is  very  faint,  or  invisible,  the  liquid  should  be  concen- 
trated to  small  bulk  for  use  as  the  test  solution,  and  placed  in  a 
small  measuring-flask  of  25,  50  or  100  c.c.,  according  to  the  depth 
of  color,  which  must  be  less  than  that  of  the  standard  solution. 
This  last  is  prepared  by  dissolving  0.25525  gram  of  normal  potas- 
sium chromate  (K^CrCU)  in  a  liter  of  water,  the  solution  containing 
then  0.0001  gram  of  CfoOa  per  cubic  centimeter. 

The  depth  of  color  of  the  test  solution  is  then  compared  with 
that  of  the  standard  exactly  as  was  done  in  the  determination  of 
titanium  dioxide  (p.  169)  or  manganese  by  the  colorimetric  method, 
a  definite  volume  of  standard  being  diluted  with  water  from  a 
burette  till  the  two  colors  are  alike.  The  results,  as  shown  by 
Hillebrand,  are  very  accurate  for  the  small  quantities  found  in 
rocks. 

The  determination  of  vanadium  is  so  seldom  necessary,  and 
the  method  is  so  complex,  that  it  need  not  be  given  here.  If  it  is 
desired  to  determine  it,  Hillebrand's  method  should  be  used,  a  full 
description  of  which  is  given  by  him.1 

It  is  to  be  remembered  that  the  vanadium,  as  V2Os,  is  to  be 
subtracted  from  the  apparent  percentage  of  P2Os,  as  it  is  precip- 
itated with  this  as  vanadomolybdate  (p.  216). 

Molybdenum  may  be  determined  in  the  portion  used  for 
vanadium.  It  is  scarcely  ever  looked  for  but  if  it  is  desired  to  do  so 
the  method  of  Hillebrand  2  may  be  used. 

24.  COPPER  AND  NICKEL  3 

If  it  is  desired  to  determine  copper,  or  other  metals  of  the 
hydrogen-sulphide  group,  which  may  rarely  be  present,  it  is  advis- 
able to  use  a  separate  portion,  rather  than  determine  them  in 
the  sulphides  remaining  after  determination  of  manganese  in 
the  main  portion,  if  the  acetate  method  has  been  used.  This 
is  chiefly  because  in  this  they  are  contaminated  with  platinum, 
and  partly  because  appreciable  amounts  of  copper  will  probably 
have  been  introduced  from  the  water-baths  (Hillebrand). 

1  Hillebrand,  pp.  148-154. 

2  Ibid.,  p.  150. 

3Cf.  p.  18, 19;   Hillebrand,  pp.  98,  116,  220. 


COPPER  AND  NICKEL  239 

The  weighed  portion,  preferably  about  2  grams,  is  decom- 
posed in  a  platinum  crucible  (placed  in  the  radiator  described  on 
p.  40)  with  sulphuric  and  hydrofluoric  acids.  When  decompo- 
sition is  complete,  as  indicated  by  the  absence  of  gritty  particles, 
the  contents  of  the  crucible  are  evaporated  to  dryness.  Com- 
plete expulsion  of  the  hydrofluoric  acid  is  insured  by  adding 
sulphuric  acid  and  again  evaporating  to  dryness,  repeating  the 
operation,  if  necessary.  The  residue  in  the  crucible  is  digested 
with  dilute  sulphuric  acid  (1 :4)  for  about  five  minutes  and  after 
a  second  dilution  (1:8)  is  filtered.  Hydrogen  sulphide  is  now 
passed  into  the  filtrate. 

The  precipitated  cupric  sulphide  is  filtered  off  rapidly  and 
washed  with  water  containing  hydrogen  sulphide.  The  filter 
containing  it  is  ignited  in  a  small  weighed  platinum  crucible, 
moistened  with  a  few  drops  of  nitric  acid,  cautiously  evaporated 
to  dryness,  ignited,  and  the  residue  is  weighed  as  CuO.  Multi- 
plication of  this  by  0.8  reduces  it  to  Cu. 

Ammonia  in  excess  is  added  to  the  filtrate,  and  a  current 
of  hydrogen  sulphide  passed  through  it  for  a  quarter  of  an  hour, 
a  few  drops  of  acetic  acid  are  added,  and  the  liquid  is  allowed  to 
stand  for  some  hours,  when  the  nickel  (and  cobalt)  sulphides  are 
caught  on  a  5J-cm.  filter,  ignited  and  weighed  as  oxide.  The 
amount  of  cobalt  in  terrestrial  rocks  is  so  small  that  it  is  not  neces- 
sary to  separate  it  from  the  nickel,  but  its  presence  may  be  estab- 
lished, if  desired,  by  testing  the  oxide  with  the  borax  bead. 

Nickel  may  also  be  determined  as  nickel  dimethyl  glyoxime,1 
either  in  the  original  slightly  acid  solution  of  the  rock  powder 
if  copper  is  not  to  be  determined,  or  in  the  filtrate  from  the  copper 
sulphide. 

The  slightly  acid  solution  is  almost  neutralized  with  ammonia, 
about  2  grams  of  tartaric  acid  added  to  hold  up  iron,  and  a  little 
acetic  acid  added.  It  is  then  heated  nearly  to  boiling  and  about 
5  or  10  c.c.  of  1  per  cent  alcoholic  solution  of  dimethyl  glyoxime 
are  added.  The  volume  of  the  alcohol  should  be  less  than  half 
that  of  the  solution  containing  the  nickel,  because  of  the  slight 
solubility  of  the  precipitate  in  alcohol.  Ammonia  water  is  added 
drop  by  drop,  with  stirring,  until  the  liquid  smells  slightly  of  it, 
and  the  voluminous  reddish  precipitate  is  filtered  hot  through  a 
1  Treadwell,  2,  pp.  129-130. 


240  METHODS 

weighed  Gooch  crucible.  It  is  washed  with  hot  water,  dried  at 
110°,  and  weighed.  The  weight  of  the  precipitate,  which  has  the 
composition  Ni(C4H7N202)2,  is  multiplied  by  the  factor  0.259 
to  reduce  it  to  NiO. 

25.  BORIC  OXIDE  l 

Boric  oxide  is  seldom,  if  ever,  determined  in  rocks,  though  it 
occurs  in  tourmaline-bearing  granites  and  in  metamorphic  rocks 
that  contain  axinite  or  dumortierite.  This  neglect  has  been  due 
chiefly  to  the  difficulties  connected  with  its  exact  determination. 
As,  however,  it  may  be  of  interest  to  determine  it  in  some  rocks,  as 
well  as  in  glasses,  which  are  essentially  silicate  material,  a  brief 
description  of  a  reliable  method  will  be  given  here. 

Chapin  found  that,  with  some  modifications,  the  Gooch- 
Rosenblaadt  method  could  be  adapted  to  the  analysis  of  minerals 
containing  boric  oxide;  and  Allen  and  Zies  find  that  Chapin's 
method  is  by  far  the  most  trustworthy  for  determining  the  boric 
oxide  content  of  glasses,  so  that,  therefore,  it  should  be  applicable  to 
rocks,  which  are  of  less  complex  composition  than  many  glasses.  For 
the  details  Chapin's  and  Allen  and  Zies'  papers  should  be  consulted. 

The  rock  powder  is  fused  with  sodium  carbonate  and  the  cake 
is  brought  into  solution  with  hydrochloric  acid,  exactly  as  with  the 
main  portion  (pp.  131  et  seq.),  except  that  the  amount  of  acid  is 
kept  as  small  as  possible.  After  dehydration  of  the  unfiltered  liquid 
by  the  addition  of  calcium  chloride,  the  boric  oxide,  freed  by  the 
hydrochloric  acid,  is  volatilized  as  methyl  borate  by  passing  the 
vapor  of  specially  pure  methyl  alcohol  through  the  hot  liquid  and  is 
collected  by  distillation.  The  solution  of  methyl  borate  in  methyl 
alcohol  so  obtained  is  treated  with  an  excess  of  sodium  hydroxide  to 
form  sodium  borate,  from  which  the  alcohol  is  distilled  off  without 
loss  of  boric  oxide.  The  excess  of  sodium  hydroxide  is  then  titrated 
with  hydrochloric  acid,  and  finally  the  free  boric  oxide  is  titrated 
with  barium  hydroxide  in  the  presence  of  mannite. 

1  Hillebrand,  pp.  199-200;  Mellor,  pp.  578-589;  Gooch,  Am.  Chem.  Jour., 
9,  p.  23,  1887;  T.  Rosenblaadt,  Zeits.  Anal.  Chem.,  26,  p.  21,  1887;  W.  H. 
Chapin,  Jour.  Am.  Chem.  Soc.,  30,  p.  1691,  1908;  Allen  and  Zies,  paper  to  be 
published  in  the  Jour.  Am.  Ceram.  Soc.,  I  am  indebted  to  Dr.  Zies  for  this 
sketch  of  the  Chapin  method,  which  he  has  used  in  the  study  of  optical 
glasses  in  this  laboratory. 


APPENDIXES 


1.  FACTORS  FOR  CALCULATION 


Constituent. 

Sought. 

Found. 

Factor. 

Logarithm. 

Baryta. 

BaO 

BaSO4 

0  66 

9  81954 

Chlorine  

C12 

AgCl 

0  247 

9  39270 

Chlorine.  .  .  . 

C12 

Ag 

0  33 

9  51851 

Copper. 

Cu 

CuO 

0  80 

9  90309 

Fluorine  

F2 

CaF2 

0  49 

9  69020 

Magnesia. 

MgO 

M£2P2O7 

0  3621 

9  55883 

Manganous  oxide  
Manganous  oxide  
Phosphorus  pentoxide  
Phosphorus  pentoxide  
Potash  

MnO 
Mn3O4 
P205 
P205 
K2O 

Mn3O4 
MnO 
Mg2P207 
24MoO3-P2O6 
K2PtCl6 

0.93 
1.08 
0.638 
0.04 
0  1934 

9.96848 
0.03342 
9.80482 
8.60206 
9  28646 

Potash  

KC1 

K2PtCl6 

0  307 

9  48714 

Potash 

K2O 

KC1O4 

0  3393 

Q  fj^n^S 

Potash  

KC1 

KC1O4 

0  538 

9  73078 

Soda  

Na2O 

NaCl 

0  5304 

9  72460 

Strontia. 

SrO 

SrSO4 

0  56 

9  74819 

Sulphur  

S 

BaSO4 

0  137 

9  13672 

Sulphur  trioxide. 

SO3 

BaSO4 

0  343 

9  53529 

Zirconia. 

ZrO2 

xZrO2-?/P2O6 

0  52 

9  71600 

To  obtain  the  weight  of  the  substance  sought,  the  weight  of  the 
substance  found  is  multiplied  by  the  appropriate  factor.  The 
factors  are  based  on  the  1916  Table  of  Atomic  Weights,1  and  are 
carried  out  only  as  far  as  is  deemed  appropriate  for  the  quantities 
usually  found  in  igneous  rocks. 

1  Clarke,  Thorpe,  and  Urbain,  Jour.  Am.  Chem.  Soc.,  38,  p.  2220,  1916. 
For  a  discussion  of  appropriate  atomic  weights  to  be  used  in  factors,  see  Mellor, 
p.  250. 

241 


242  APPENDIXES 

2.  EXAMPLE  OF  ANALYSIS 

The  rock  analyzed  is  a  gray  porphyritic  basaltic  lava  from 
Mt.  Etna  showing  in  the  hand  specimen  conspicuous  augite  and 
olivine  phenocrysts  about  5  mm.  in  diameter,  and  less  conspicu- 
ous but  more  numerous  crystals  of  plagioclase.  Microscopically 
the  plagioclase  is  found  to  be  about  Ab2Ans  to  AbiAn2.  The 
same  minerals  are  represented  in  the  holocrystalline  groundmass 
together  with  necessary  magnetite  and  apatite. 

The  frontispiece  is  from  a  drawing  representing  the  summit 
of  Mt.  Etna  about  100  years  after  this  lava  was  extruded. 

There  are  given  the  partial  results  of  an  analysis  that  was  made 
to  check  up  the  times  taken  for  the  various  determinations  and 
operations.  The  analysis  was  begun  on  June  12,  at  8:30  A.M.  and 
the  determinations  given  here  were  completed  by  noon  of  June  16, 
though  there  were  several  interruptions.  The  working  day  was 
from  9  A.M.  to  4:30  P.M.  The  calculations  are  presented  in  com- 
mon arithmetical  form,  though  (five-place)  logarithms  may  be 
used,  and  will  save  time  and  space. 

The  results  of  the  complete  analysis  are  as  follows: 

BASALT.    LAVA  OF   1669.    CATANIA,   MT.  'ETNA 

SiO2 49.62 

A12O3 16.00 

Fe2O3 2.81 

FeO 7.61 

MgO 5.20 

CaO 10.25 

Na20 4.12 

K20 1.46 

H2O+ 0.22 

H2O- 0.07 

CO2 1 none 

TiO2 1.64 

ZrO2 1 none 

P2O5 0.62 

S1 0.05 

Cr2O3 1 none 

MnO 0.13 

BaO1 0.09 

SrO 0.03 

99.92 
1  These  determinations  are  not  given  in  the  example. 


APPENDIXES 


243 


Si02, 
49.62 


H20+, 
0.22 


H2O-, 
0.07 


MnO, 
0.13 


E,  10.    BASALT.    LAVA  OF  1669.    CATANIA,  MT.  ETNA 
SiO2,  H2O+,  H2O-,  MnO. 

Cruc.  +subst.      =  33 . 0909 
Cruc.  (palau)      =32.0712 
Main  portion  l    =   1 . 0197 
Cruc.  +SiO2+z  =  25. 0185          Cruc.+SiO2+z  =25.0185 


Cruc. 


Cruc.+extra 
Cruc.+z 
Extra  SiO22 


=  24.5101 


.5084 


Cruc.+z 
Main  SiO2 
Extra  SiO2 


=  24.5176 
=24.5142 
=  .0034 


=  24.5159 
=  .5026 
=  .0034 

1. 0197). 50600(. 4962 
40788 
98120 
91773 


Tube +subst.=  28. 7643 
Tube  =28.2492 

Subst.  taken  =      .5151 

Total  H2O  =  0.29 
H2O-        =0.07 
H2O+        =0.22 
Cruc.  +subst.  =  33 . 281 1 
Cruc.  =32.0700 

1.2111 


63470 
61182 

22880 

Tube +H2O  =  21. 1212 
Tube  -  H2O =21.1197 

. 515). 001500(. 0029 
1030 
4700 
4635 


Cruc.  +subst.  =  33 . 281 1 
Dried  at  110°  =  33. 2802 

1.21).000900(.0007 

847 
53 


Same  portion,  test  solution  diluted  to  200  c.c. 

H2O  26. 03).  00020000(.  000007683 


10  cc. 


15.8 

16.3 

16.0 

3)48T 

10+16.03=26.03 


1.21).0015366(.0013 
121 
326 


18221 
17790 
15618 
21720 
20824 
8960 
7809 


200 


.001536600 


1  The  cold  cake  was  almost  colorless,  with  brown  patches  of  iron  carbonate,  showing  that 
very  little  manganese  was  present. 

2  This  is  the  SiO?  recovered  from  the  alumina  precipitate;  see  next  page. 


244 


APPENDIXES 


A1203, 
16.00 

Fe203, 
2.81 

E.  10.     A12O3,  Fe2O3,  TiO2, 
Cruc.+Al2O3+etc.1  =  24.8161          A12O3, 
Cruc.                        =24.5101          Fe2O3 

etc.  =  .3060 
=  .1148 

.3060 
Ti02 
Burette  +KMnO4    =  178  .  14 
Burette-KMnO4    =131.73            giQ2 

.1912 
.0167 

.1745 
.0034 

46.41 

P206 
Fe2O3  value2            =.002473 

46'41           Art 

.1711 
.0063 
.1648 
.0003 

2473 

!4883f             Mn*°< 

9892 

.1645 
.0013 

0197).  16320(.  1600 
10197 

Total  Fe2O3          =.11477193 
FeO  as  Fe2O3  3      =  .  08616465 

61230 
61182 

1  .0197)  .02860728(.  02805+ 
20394 
82132 
81576 

4800 

o  500  c.c. 
.00074000(.  000033439 
6639                      500 

55600 
50985 

4615 
Sol.  from  Fe203,  test  solution  diluted  t 
^TiO2,            H2O                           22.13) 
10  cc.            11.8 
12.5 
12.1 

Ti02. 
1.64 

7610      .0167195 
6639 
9710 

8852 
8580 
6639 

3)36.4 

10+12.13=22.13 
1.  0197).  0167195(.  01639 
10197 

65225 
61182 

19410 
19917 

40430 
30591 

98390 

1  A  little  persulphate  was  added  before  precipitation  with  ammonia  to  throw  down  the 
manganese  with  the  alumina,  though  this  would  generally  not  be   necessary  with  the  very 
small  amount  of  manganese  present. 

2  One  gram  of  the  permanganate  solution  =.002473  gram  FezOs  or  .002228  gram  FeO. 

3  For  the  calculation  of  this  see  p.  245. 


APPENDIXES  245 


CaO, 
10.25 

SrO, 
0.03 

MgO, 
5.20 

E.  10.     CaO,  SrO,  MgO,  FeO, 
Cruc.  +  (Ca,  Sr)O  =  19  .  8538 
Cruc.                     =19.7490 

Cruc.  +SrSO4  =  19  .  7495 
Cruc.               =19.7490 

(Ca,  Sr)O              =      .  1048 
1.  0197).  104800(.  1028 
10197 
28300 
20394     (Ca,Sr)O  =  10 

SrSO4              =     .0005 
.56 

.30 
25 

•28%                  SrO=    .000280 
.03 

79060       SrO 

10 

Cruc.  +Mg2P2O7  =24.2078 
Cruc.                     =24.0614 

.25 
.3621 
.1464 

14484 
21726 
14484 
3621 

Mg2P2O7                =     .  1464 

Cruc.  +subst.  =  32  .  5919 
Cruc.               =32.0700 

1.0197).  05301  144  (.05198 
50985 

0264 
10197 
100670 
91773 
88970 

Burette  +  KMnO4  =  176  .  94 
Burette  -KMnO4  =  159.  11 

FeO, 
7.61 

.5219 

.0022281 
17.83 

17.83 
.0024732                       1.01973 
17.83                          .0845 

6684 
17824 
15596 

2228 

7419                         50985 
19784                         40788 
17311                         81576 

2473                        .08616465 

.  5219)  .  03972524(  .0761         .  5219) 

.04409359(.  0845 

1  The  calculation  of  the  FeO  value  of  the  permanganate. 

2  The  calculation  of  the  FezOs  value  of  the  permanganate. 

3  The  calculation  of  the  FeO  as  FezOs  in  terms  of  percentage  of  the  main  portion,  for  Bub- 
traction  from  the  total  iron  oxides  (p.  244). 


246  APPENDIXES 


K20, 
1.46 


Na20, 
412 


0.62 


E.  10.    ALKALIES, 

Tube +subst.  =  23 . 1212  Cruc. +NaCl  +KC1 = 30 .0955 

Tube  -  subst .  =  22.5639  Cruc.                        =30.0393 

.5573  .05622 

Gooch+K2PtCl6  =  23.1475  NaCl+KCl              =  .0562 

Gooch                  =23.1054  KC1                          =.0129 

K2PtCl6               =     .0421  NaCl                         =.0433 

.1934  .0421                                  .5304 

.0421  .307                                   .0433 


1934  2947  15912 

3868  12630  15912 

7736  .0129247  21216 


.  5573) .  00814214( .  0146  . 5573) .  02296632( .  0412 
5573  22292 

25691  6743 

22292  5573 


33990  11700 

Pt.  basin+subst.  =22.3397  Cruc. +Mg2P2O7= 24. 0725 

Pt.  basin  =21.1965  Cruc.  =24.0614 

1.1432  .0111 

.0111 
.638 


888 
333 
666 


1.143).  007081  (.0062 

6858 
~2238 


1  On  this  page  can  also  be  recorded  the  data  for  BaO,  S,  and  ZrCh,  which  are  omitted  here. 

2  The  weight  of  the  mixed  chlorides  is  very  nearly  the  combined  percentages  of  Na2O  and 
K2O  (p.  84) ;  the  presence  of  potassium  chloride,  with  the  higher  atomic  weight  of  potas- 
sium, tends  to  make  up  for  the  weight  of  substance  taken  being  more  than  .5304  gram. 


APPENDIXES  247 


3.  REFERENCES 

The  following  works  have  been  consulted  and  some  of  them  are 
often  cited.1  In  the  references,  as  a  general  rule,  only  the  author's 
name  and  page  number  are  given. 

CLASSEN,  A.,  Ausgewahlte  Methoden  der  Analytischen  Chemie.  Braunschweig, 
Vol.  I,  1901  (the  metallic  elements);  Vol.  II,  1903  (the  non-metallic 
elements).  This  large  and  useful  book  contains  good  descriptions  of 
well-selected  methods,  though  some  have  been  now  superseded. 

DITTRICH,  M.,  Anleitung  zur  Gesteinsanalyse.  Leipzig,  1905,  98  pp.  A 
small  book  that  gives  some  details  of  manipulation,  but  is  not  up  to  date, 
and  neglects  the  minor  constituents.  This  is  the  book  referred  to  in  the 
text  under  "  Dittrich." 

DITTRICH,  M.,  Analytische  Methoden  der  Silikate.  In  Doelter,  Handbuch 
der  Mineralchemie,  Vol.  I,  pp.  560-594,  1912.  A  condensation  of  the  pre- 
ceding book.  It  is  very  summary  and  gives  few  details  or  new  methods. 

FRESENIUS,  R.,  Quantitative  Chemical  Analysis.  Translation  of  the  sixth 
German  edition  by  A.  I.  Cohn,  New  York,  1904.  Vol.  I,  780  pp;  Vol.  II, 
1255  pp.  The  first  volume  deals  with  general  inorganic  analysis;  the 
second  with  organic  analysis  and  many  special  and  commercial  methods. 
A  reprint  of  Hillebrand's  Bulletin  176  is  also  included.  This  is  a  classical, 
well-known,  and  useful  work,  but  the  translation  gives  few  of  the 
modern  methods  and  is  somewhat  antiquated.  The  treatment  of  silicate 
analysis  is  very  inadequate  and  unsatisfactory. 

GOOCH,  F.  A.,  Representative  Procedures  in  Quantitative  Chemical  Analysis. 
New  York,  1916,  262  pp.  This  book  describes  many  of  the  analytical 
operations  in  considerable  detail.  About  one-half  of  it  is  devoted  to 
volumetric  analysis,  and  the  analysis  of  silicates  is  treated  very  inad- 
equately. 

HILLEBRAND,  W.  F.,  The  Analysis  of  Silicate  and  Carbonate  Rocks.  U.  S. 
Geological  Survey  Bulletin  422,  239  pp.,  1910.  Reprint  1916.  This 
excellent  and  classical  treatise  is  the  best  book  on  the  subject.  It  is 
written  for  the  experienced  chemist  rather  than  for  the  beginner.  The 
reprint  differs  from  the  edition  of  1910  only  in  verbal  corrections  of  minor 
importance.  Hillebrand's  earlier  Bulletins  2  on  rock  analysis  will  not 
often  be  referred  to  in  this  book. 

JANNASCH,  P.,  Praktischer  Leitfaden  der  Gewichtsanalyse.  2te.  Aufgabe, 
Leipzig,  1904,  450  pp.  Only  partly  devoted  to  silicate  analysis.  Sev- 
eral new  methods  devised  by  the  author  are  described,  but  the  book 
is  unsatisfactory. 

1  For  a  fuller  list  see  Mellor,  p.  733. 

2  No.  176,  1900;  No.  305,  1907. 


248  APPENDIXES 

MELLOR,  J.  W.,  A  Treatise  on  Quantitative  Inorganic  Analysis.  London, 
1913,  778  pp.  This  excellent  work  is  replete  with  references,  descriptions 
of  alternative  methods,  and  useful  hints  and  tables.  It  is  devoted  largely 
to  the  analysis  of  silicates,  including  glazes,  and  will  be  found  very  useful 
to  every  analyst  as  a  handbook  for  reference. 

MORSE,  H.  N.,  Exercises  in  Quantitative  Chemistry.  Boston,  1905,  556  pp. 
Being  a  selection  of  exercises,  only  a  few  of  those  needed  by  the  rock 
analyst  will  be  found  in  this  book,  but  some  of  these  are  described  well  and 
in  detail.  The  analysis  of  silicates  is  somewhat  inadequately  treated. 

NOYES,  A.  A.,  AND  OTHERS.,  A  system  of  Qualitative  Analysis.  Boston, 
1906-1914.  This  is  a  separate  publication  of  a  series  of  papers  that 
appeared  in  the  Technological  Quarterly  and  The  Journal  of  the  American 
Chemical  Society.  It  gives  the  details  and  results  of  numerous  experi- 
mental researches  and,  while  primarily  adapted  to  qualitative  analysis, 
will  give  some  useful  information  as  to  quantitative  methods. 

OSTWALD,  W.,  The  Scientific  Foundations  of  Analytical  Chemistry.  London, 
third  edition,  1908.  An  excellent,  short  and  lucid  discussion  of  the  appli- 
cation of  the  principles  of  physical  chemistry  to  the  problems  of  analysis. 
The  fifth  German  edition  (Die  Wissenschaftlichen  Grundlagen  der 
Analytischen  Chemie)  appeared  in  1910. 

STIEGLITZ,  J.,  The  Elements  of  Qualitative  Chemical  Analysis;  Vol.  I,  Fun- 
damental Principles  and  their  Application.  New  York,  1913,  312  pp. 
This  excellent  book  resembles  that  of  Ostwald  in  presenting  the  application 
of  physical  chemistry  to  the  methods  of  analysis.  Though  treating 
primarily  of  qualitative  analysis,  it  is  almost  equally  applicable  to  quan- 
titative methods,  and  its  discussions  will  be  found  very  useful  to  the 
student. 

TREAD  WELL,  F.  P.,  Analytical  Chemistry;  Vol.  I,  Qualitative  Analysis,  Vol. 
II,  Quantitative  Analysis.  Translation  by  W.  T.  Hall,  New  York,  1916 
and  1911.  This  well-known  and  very  useful  work,  much  superior  to 
Fresenius,  contains  many  recent  methods,  and  should  be  in  the  hands  of 
every  analyst,  though,  like  many  others,  its  treatment  of  silicate  analysis 
is  too  brief  and  little-detailed  to  be  very  serviceable  to  the  student  who  is 
working  alone. 

WASHINGTON,  H.  S.,  Chemical  Analyses  of  Igneous  Rocks.  U.  S.  Geological 
Survey  Professional  Paper  99.  Washington,  1917,  1201  pp.  This  col- 
lection of  chemical  analyses  of  rocks  is  accompanied  by  a  text,  in  which 
are  discussed  such  topics  as  the  character  of  rock  analyses,  including  their 
accuracy,  completeness,  and  representativeness,  and  the  criteria  by  which 
to  judge  of  their  quality.  It  also  includes  a  description  of  the  quanti- 
tative classification  of  igneous  rocks  and  the  methods  used  in  calculating 
the  mineral  composition.  The  large  number  of  analyses  recorded  will 
give  the  student  many  examples  of  excellence  to  be  emulated,  as  well  as 
many  "  horrible  examples  "  that  show  to  what  depths  the  careless  and 
incompetent  analyst  can  descend. 


INDEX  OF  AUTHORS 


PAGE 

ADAMS  (and  JOHNSTON),  Precipitation  of  barium  sulphate 227 

ALLEN,  E.  T.,  Abrasion  of  mortar 66 

Adsorption  of  water  by  powder 66,  72 

ALLEN  (and  DAY),  Adsorption  of  water  by  powder 66,  72,  209 

ALLEN  and  JOHNSTON,  Action  of  ammonia  on  glass 48 

Precipitation  of  barium  sulphate 227 

ARTHUR  H.  THOMAS  Co.,  Catalogue 27 

AUSTIN  (and  GOOCH),  Ammonium  magnesium  phosphate 180 

BARNEBY,  O.  L.,  Boric  acid  in  ferrous  oxide  determination 186 

BAXTER  and  KOBAYASHI,  Determination  of  potash  as  perchlorate 207 

BAYLEY  (and  HICKS),  Determination  of  potash 208 

BLAKE  (and  GOOCH),  Determination  of  potash  as  perchlorate 207 

BLUM,  W.,  Hygroscopic  character  of  alumina 158 

Precipitation  of  alumina 150,  151 

Sodium  oxalate  as  standard 53 

Standardization  of  permanganate  solution ' .  .  .  .     53 

BURGESS,  G.  K.,  Quality  of  platinum 31,  32 

CAIN  and  HOSTETTER,  Precipitation  of  vanadium  with  phosphorus 216 

CHAPIN,  W.  H.,.  Method  for  boric  oxide 240 

CHEMICAL  SOCIETY  OF  LONDON,  Use  of  "  platinichloride  " 202 

CIRKEL,  F.,  Asbestos 49 

CLARKE,  F.  W.,  Average  igneous  rock 125 

Occurrence  of  the  elements 17 

Occurrence  of  titanium 9,  1 7 

Table  of  atomic  weights 241 

CLASSEN,  A.,  "  Ausgewahlte  Methoden" 247 

COHN,  A.  I.,  Translation  of  Fresenius 247 

CONNOR,  M.  F.,  Action  of  nitric  acid  on  magnesium  pyrophosphate 181 

Contamination  of  sample  by  silica 66 

Errors  in  determination  of  the  alkalies 193 

Summation  of  analyses 126 

COOKE,.  Method  for  ferrous  oxide  determination 183 

DANA,  E.  S.,  Asbestos 49 

DAUDT,  H.  W.,  Precipitation  of  alumina 150' 

249 


250  INDEX  OF  AUTHORS 

»AGE 

DAUDT,  H.  W.,  Volatilization  of  chlorides  from  alumina 149 

DAY  and  ALLEN,  Adsorption  of  water  by  powder 66,  72,  209 

DITTRICH,  M.,  "  Anleitung  zur  Gesteinsanalyse " 247 

"Analytische  Methoden" 247 

Determination  of  alkalies 192 

Errors,  study  of  analytical 120,  124 

Fluorine  in  ferrous  oxide  determination 186 

Macerated  paper 154 

Quartz,  use  of,  in  ferrous  oxide  determination 189 

Rare  earths,  determination  of 229 

DOELTER,  C.,  "Handbuch  der  Mineralchemie  " 247 

DUNNINGTON,  E.  P.,  Determination  of  titanium 169 

EIMER  &  AMEND,  Catalogue 27 

FAY,  H.,  Determination  of  titanium 175 

FRESENIUS,  R.,  " Quantitative  Analysis" 247 

GAGE,  R.  B.,  Use  of  calcium  phosphate  in  determination  of  ferrous  oxide  185 

GOOCH,  F.  A.,  Method  for  boric  oxide 240 

Method  for  lithia 207 

Method  for  titanium 175 

Position  of  crucible  in  ignition 104 

"Procedures  in  Quantitative  Analysis 247 

GOOCH  and  AUSTIN,  Ammonium  magnesium  phosphate 180 

GOOCH  and  BLAKE,  Perchlorate  method  for  potash 207 

GOOCH  and  NEWTON,  Oxidation  of  titanium  by  copper  sulphate 163 

HALL,  W.  T.,  Translation  of  Treadwell 248 

HEMPEL,  Contamination  of  sample  by  silica 65 

HICKS  and  BAYLEY,  Determination  of  potash 208 

HILLEBRAND,  W.  F.,  Abrasion  of  mortar 65,  6(5 

Adsorption  of  water  by  rock  powder 66,  72,  209 

Air-dry  powder,  use  of 72 

Analyses,  completeness  of 9 

errors  in 124 

summation  of 126,  127,  128 

time  needed  for 113 

"Analysis  of  Silicate  and  Carbonate  Rocks" 247 

Barium,  distribution  of 9 

"Bulletin  422" 247 

Contamination  of  sample  by  iron 65,  67 

Fluorine,  influence  of,  in  titanium  determination . .  .   169 

Limits  of  error 124 

Manganese,  distribution  of,  in  analysis 14 


INDEX  OF  AUTHORS  251 

PAGE 

HILLEBRAND.  W  F.,  Occurrence  of  the  elements 9,  17 

Oxidation  of  ferrous  oxide  in  grinding 183 

Pulverization  of  sample 64,  65 

Radiator 145 

Report  on  analysis  of  limestone 121 

Silica,  double  evaporation  of 139 

Water,  combined  and  hygroscopic 12,  209 

HOLMES,  A.,  Occurrence  of  radium 21 

HOSTETTER  (CAIN  and),  Precipitation  of  vanadium  with  phosphorus.  ...  2 It 

HOSTETTER  (SOSMAN  and),  Oxidation  of  ferrous  oxide  and  grinding 183 

Oxidation  of  magnetite  to  hematite 212 

HOWE  (PENFIELD  and),  Use  of  lead  oxide  in  water  determination 215 

HUNTER,  Error  in  determination  of  the  alkalies 193 

IDDINGS,  J.  P.,  Occurrence  of  the  elements 17 

JANNASCH,  P.,  "Praktischer  Leitfaden  der  Gewichtsanalyse  " 247 

JOHNSTON,  J.,  Loss  of  carbon  dioxide  in  grinding 66 

JOHNSTON  and  ADAMS,  Precipitation  of  barium  sulphate 227 

JOHNSTON  (ALLEN  and),  Action  of  ammonia  on  glass 48 

Precipitation  of  barium  sulphate 227 

KEMP,  J.  F.,  Occurrence  of  the  elements 17 

KOBAYASHI  (BAXTER  and),  Perchlorate  method  for  potash 207 

KRAUCH,  C.,  Testing  of  reagents • 45 

KRAYER,  P.  J.,  Balance 27 

Weights 30 

KREIDER,-J.  L.,  Apparatus  for  carbon  dioxide. 237 

LENHER  and  TRUOG,  Determination  of  silica 139,  142 

MAUZELIUS,  R.,  Oxidation  of  ferrous  oxide  in  grinding 183 

McBRiDE,  R.  S.,  Standardization  of  permanganate 53 

Weighing  burette 35,  107 

MELLOR,  J.  W.,  "Quantitative  Inorganic  Analysis" 248 

MERCK,  E.,  Testing  of  reagents 45 

MERRILL,  G.  P.,  Rock  weathering 59,  61 

MERWIN,  H.  E.,  Color  perception 172 

Determination  of  fluorine 235 

Determination  of  titanium 124,  168,  169 

Titanium  solution,  bleaching  effect  of  alkali  sul- 
phates     169 

MINOR  (PENFIELD  and),  Determination  of  fluorine 233,  235 

MITSCHERLICH,  Method  for  ferrous  oxide 183 

MOROZEWICZ,  R.,  Determination  of  rare  earths 229 


252  INDEX  OF  AUTHORS 

PAGE 

MOROZEWICZ,  R.,  Sodium  platinichloride 204 

MORSE,  H.  N.,  " Exercises  in  Quantitative  Chemistry" 248 

NEUBAUER,  Ammonium  magnesium  phosphate 180 

NOTES,  A.  A.,  "Qualitative  Analysis" 248 

OSTWALD,  W.,  "Foundations  of  Analytical  Chemistry" 248 

PENFIELD,  S.  L.,  Analyses  by 125 

Colorimetric  method  for  titanium 174 

Method  for  water 210,  213 

PENFIELD  and  HOWE,  Use  of  lead  oxide  in  determination  of  water 215 

PENFIELD  and  MINOR,  Determination  of  fluorine 233,  235 

PRATT,  J.  H.,  Determination  of  ferrous  oxide 183,  190 

RICHARDS,  T.  W.,  Precise  methods 75 

RIDGWAY,  R.,  Color  Standards 170 

RIPPER,  Weighing  burette 31, 107 

ROBERTS,  H.  S.,  Macerated  paper 154 

ROBINSON,  H.  H.,  Analysists  and  summations 126 

Direction  of  error 121 

ROSENBLAADT,  T.,  Method  for  boric  oxide 240 

ROSIWAL,  Method  for  estimating  mineral  composition 7 

SANDBERGER,  Occurrence  of  elements 10 

SCHREINER,  O.,  Colorimeter 43,  174 

SMITH,  J.  L.,  Determination  of  alkalies 191,  192,  193 

SOSMAN  and  HOSTETTER,  Oxidation  of  ferrous  oxide  in  grinding 183 

Oxidation  of  magnetite  to  hematite 212 

SPENCER,  Law  of  evolution 1 

STEIGER,  G.,  Colorimeter 43,  175 

Determination  of  fluorine 169,  235 

Distribution  of  manganese  in  analysis 14 

STIEGLITZ,  J.,  "Qualitative  Chemical  Analysis" 24.8 

THOMAS  Co.,  ARTHUR  H.,  Catalogue 27 

THORNTON,  W.  M.,  Method  for  titanium 176 

Separation  of  alumina,  iron,  and  titanium 162 

TREADWELL,  F.  P.,  "Analytical  Chemistry" 248 

VOGT,  J.  H.  L.,  Occurrence  of  the  elements 10,  17 

WALKER  and  SMITHER,  Quality  of  glassware 34 

WALTERS,  H.  E.,  Colorimetric  method  for  manganese 220 

WARREN,  C.  H.,  Determination  of  titanium  dioxide 175,  176 

WASHBURN,  E.  W.,  Weighing  burette 35,  107 


INDEX  OF  AUTHORS  253 

PAGE 

WASHINGTON,  H.  S.,  "Collection  of  Analyses  of  Igneous  Rocks,"  Pro- 
fessional Paper  99 248 

WELLER,  Colorimetric  method  for  titanium 168 

WILLIAMS,  I.  A.,  Brittleness  of  minerals 67,  71 

WINTER,  Cost  of  platinichloride  determination 202 

WOY,  Determination  of  phosphorus  pentoxide 217 

ZALESKI,  S.,  Brittleness  of  minerals 67 

ZIES,  E.  G.,  Decomposition  of  sodium  carbonate 146 

Method  for  boric  oxide .   240 


INDEX  OF  SUBJECTS 


Abrasion  of  mortar 65,  66 

Accuracy  of  analyses 3 

Acid,  acetic,  reagent 156 

use  of 156 

hydrochloric,  reagent 47 

hydrofluoric,  reagent 47 

nitric,  reagent 47 

sulphuric,  reagent 47 

Acids,  solution  of  rock  powder  in ,  . .  .     87 

Addition  method  of  weighing 129 

Adsorption  of  salts  by  precipitates 88,  90 

of  water  by  rock  powder 66,  72,  76,  208 

Aegirite,  ferrous  oxide  in 184 

Agate  mortar 43,  65,  66,  70 

Air-dry  powder,  use  of 72 

Alcohol,  addition  of 89,  141,  178,  181 

reagent 47,  204 

Alkali  chlorides,  drying  of 193,  200,  201 

sulphates,  bleaching  effects  of 169,  173 

Alkalies,  determination  of 110,  116,  191,  207,  246 

errors  in  determination  of 123,  192 

Smith  method  for 110,  191,  192,  193-207 

See  Potash  and  Soda. 

Allowable  limits  of  error 124-126 

Alteration  of  rocks 59,  60,  61 

Alumina,  determination  of 109,  146-162,  244 

errors  in  determination  of 121,  147-150 

fusion  of,  with  pyrosulphate 117,  159-162 

fusion  of,  with  sodium  hydroxide 150 

hygroscopic  character  of 158 

ignition  of 157-159 

precipitation  of 117,  146-157 

Ammonia  water,  precipitation  of  alumina  by 117,  146-155 

reage.xc 48, 148,  149,  178 

Ammonium  bisulphite,  use  of 163 

carbonate,  reagent 47 

use  of,  in  alkali  determination 198 

chloride,  presence  of  in  alumina  precipitation 148,  150 

255 


256  INDEX  OF  SUBJECTS 

PAGE 

Ammonium  chloride,  reagent 48 

use  of  in  alkali  determination 191,  195,  196 

hydroxide,  reagent 48,  148,  149,  178 

magnesium  phosphate 115,  180,  181,  216 

molybdate 48,  216 

nitrate 49,  217 

oxalate 49,  178 

persulphate 49,  151 

phosphomolybdate 216,  218 

salts,  necessity  for  presence  of 148,  150 

Amount  of  material  needed  for  analysis 62-63 

Amphoteric  character  of  alumina  precipitate 147 

Analyses,  accuracy  of 3 

allowable  limits  of  error  in 124-126 

amount  of  rock  needed  for 62-63 

character  of 3-5 

completeness  of 5,  7-11 

constituents  to  be  determined 5,  7-17 

course  of 109-113,  116-118 

"doctoring"  of 26,  61 

example  of 11&-118,  242-246 

examples  illustrating  character  of 125 

importance  of 1-3 

plan  of 109-113,  116-118 

portions  to  be  used  for 109-113 

preparation  of  sample  for 63-72 

recalculation  of,  to  100  per  cent 26,  61 

selection  of  specimen  for 57-61 

statement  of 21-26 

summation  of 8,  126-129 

time  needed  for  making 113-118 

Analysis  of  basalt 242 

Analyst,  qualifications  of 4-5,  77,  114 

Analysts,  women 5 

Analytical  methods,  errors  in 119-129 

general  discussion  of 109-113 

"Analyzed"  reagents 45 

Apparatus,  errors  caused  by 77 

fused  silica 39 

glass 34-39 

list  of 27-45 

metal 40-43 

miscellaneous 43-45 

platinum 30-34 

porcelain 39-40 

rubber 40 


INDEX  OF  SUBJECTS  257 

PAGE 

Apron,  wearing  of 74 

Asbestos 49,  99 

Ash  of  filter  paper 44,  105,  159 

Atomic  weight  determinations,  methods  used  in 75 

weights,  table  of 241 

Average  igneous  rock 125 

Balance,  character  and  care  of 27-30,  79-82 

use  of 79-84,  129-131 

zero  point  of 28,  81-82 

Balance-case. 28,  81 

Barium  chloride,  reagent 50 

Barium,  occurrence  of 9,  15,  19 

Barium  sulphate,  precipitation  of 226,  229,  231 

Baryta,  determination  of 15,  111,  225,  229 

Basalt,  analysis  of 242 

Basic  acetate  method :  . .  .   14,  124,  149,  155-157,  220 

Basin,  fused  silica 39,  199 

glass 140 

platinum 31,  136,  140,  194 

porcelain 140 

Beakers 34,  39 

Beginners 74,  76,  83,  91,  97,  114,  126,  182,  195 

Beryllium,  see  Glucinum. 

Blair's  tongs 32,  188 

Blast  burner 40 

Blast,  temperature  of 31 

Blasting  precipitates 105,  158 

Blowpipe  forceps 32 

Bolting  cloth,  silk,  for  sieve 45,  68 

Boric  acid,  reagent 50 

use  of,  in  ferrous  oxide  determination 186 

oxide,  determination  of 240 

Boron,  occurrence  of 21 

Bottles  coated  with  ceresine 46,  48,  148 

ceresine 46,  48,  148 

for  reagents 46 

Box  for  colorimeter 44 

Brittleness  of  minerals 66,  71 

Bureau  of  Standards 32,  53,  55 

Burette 34,  166,  171 

Burette-stand 43 

Burners 40 

flames  of 78,  103,  104,  188 

Cake,  color  of 135,  137 

removal  of,  from  crucible 134,  135,  161 


258  INDEX  OF  SUBJECTS 

PAGE 

Cake,  solution  of 137,  161 

Calcium  carbonate,  reagent 50 

use  of,  in  determination  of  alkalies 192,  196 

fluoride,  precipitation  of 234 

oxalate,  precipitation  of 117,  178 

oxide,  see  Lime. 

phosphate,  use  of,  in  ferrous  oxide  determination 185 

Calculation  of  analyses,  example  of 243-246 

factors  for 241 

to  be  carried  to  four  decimals 25 

to  100  per  cent 26,  61 

Caps  for  reagent  bottles 46 

Carbon  dioxide,  determination  of , 16,  112,  235-237 

examination  of  rock  powder  for 235 

generator 36 

loss  of,  on  grinding 66 

occurrence  of 15,  61 

Carbon  filter  tube 35,  99 

Carbonate,  calcium,  fusion  with 116,  193-196 

sodium,  fusion  with 85,  116,  131-137 

Care  of  balance 27-30,  79-82 

platinum 32-34,  79,  132,  135 

Carelessness,  errors  due  to 4,  79,  120 

Casseroles 39 

Catalogues,  dealers' 27 

Ceresine  bottles 46,  48,  148 

Cerium  dioxide,  determination  of 229-231 

occurrence  of 18 

Character  of  analyses 3-5 

analyst 4-5,  77,  114 

errors 75,  119 

Chlorides,  removal  of,  from  ammonia  precipitate .  .  . 149 

Chlorine,  determination  of 13,  15,  112,  232-233 

occurrence  of 13,  20 

oxygen  equivalent  of 128 

testing  of  nitrate  for 97 

Chloroplatinate,  use  of  term 202 

Chloroplatinic  acid,  solution  of 50,  203 

Chromium,  determination  of 14,  112,  226,  237-238 

occurrence  of 18 

standard  solution  of 50,  238 

Clamps 40 

Cleanliness,  necessity  for 73,  114 

Cobalt,  determination  of 15,  224 

occurrence  of 15,  18 

Cobaltinitrite  method  for  potash . 203,  208 


INDEX  OF  SUBJECTS  259 

PAGD 

Colloidal  solutions 90,  142,  148,  152 

Color  of  fused  cake 135,  137,  243 

permanganate  solution 53,  167,  185,  222 

titanium  solution 168,  171 

perception  of 76,  169,  172 

standards,  Ridgway  on 170 

Colorimeter,  Schreiner's 43,  169,  174 

Steiger's 43,  169,  175 

usual  form,  described 43,  44,  169 

use  of • 169-175 

Colorimetric  method  for 'determining  chromium 237 

fluorine 235 

manganese 220-223 

titanium 168-175 

Combined  water,  determination  of 12,  210-216 

Committee  on  analysis,  report  of 121,  124,  125 

Completeness  of  analyses 5,  7-17 

Conditions  of  laboratory,  unfavorable 76 

Cone,  platinum 32,  98 

Constituents,  list  of 11 

main 11-13 

minor 13-17 

number  of,  to  be  determined 7-17 

order  of,  in  statement  of  analyses 21-26 

Contamination  of  sample  in  pulverization 65-68 

Cooke's  method  for  ferrous  oxide 183 

Copper,  determination  of 15,  224,  238 

occurrence  of 15,  19 

Cost  of  platinichloride  determination 202 

Course  of  analysis 109-113,  116-118 

"C.  P."  reagents 46 

"Creeping" 92,  145,  160,  182 

Crucible,  care  of 32-34,  79,  132,  135 

Gooch 31,  99-101,  182,  204 

nickel 42,  145 

platinum 32,  79,  132,  135 

porcelain 40,  145 

position  of,  during  ignition 1C3,  104,  144,  158 

weighing  of 80,  82,  116 

Crushing  rock,  methods  for 64-72 

Cupferron,  reagent .  .  . 162,  177 

Decantation ,  .     93 

Decimals,  calculations  carried  to  four 25 

Decomposition,  methods  of.. .  .84-87,  131,  183,  187,  193,  217,221,  232,  234,  236 
"Decomposition/'  use  of  term 84 


260  INDEX  OF  SUBJECTS 

PAGE 

Desiccator 35,  82 

Deterioration  of  standard  solutions 54,  77,  163 

Dimethylglyoxime,  reagent 50,  239 

"  Dioxogen,"  reagent 50 

Direction  of  errors 120-124 

Distribution  of  manganese  in  analysis 14,  220 

"Doctoring"  of  analyses 26,  61 

Double  evaporation  to  render  silica  insoluble 139,  143 

precipitation,  necessity  for 90,  148,  153,  178,  181 

Drying  cylinders 35 

precipitates 101-103,  104,  144,  157,  205 

oven 42,  101 

tubes 36 

Duplicate  determinations 125,  126 

Dust,  loss  of,  in  preparing  sample 67 

errors  from 67,  73,  87,  120,  127,  129 

Earths,  rare,  determination  of 225,  229-231 

occurrence  of 18 

Electrolytes,  presence  of,  in  washing  precipitates 90,  142,  148 

Elements,  occurrence  of  the 9,  10,  17-21 

Ellis  steel  mortar 41 

End-point  in  permanganate  titration 53,  122,  167,  185,  189 

Erdmann's  float 35 

Errors,  allowable  limits  of 124-126 

character  of 75-79,  119-124 

direction  of 120-124,  127 

limits  of 124-126 

methodic 119-126,  139,  147,  162,  168,  177,  180,  183,  192,  210,  216 

numerical 74,  77 

operative 75-79,  119,  120,  127 

personal 119 

plus  and  minus 120-124,  127 

sources  of 75-79,  119-124,  127 

"Estimate,"  use  of  term 109 

Evaporating  dishes 40 

Evaporation  of  sulphuric  acid 87,  145 

to  render  silica  insoluble 139,  140 

Evolution,  law  of,  applied  to  rocks 1 

Example  of  analysis 242-246 

Excess  of  precipitant,  meaning  of  term 88 

Factors  for  calculation 206,  241 

Ferric  oxide,  determination  of 11,  110,  117,  162-167,  244 

errors  in  determination  of 122,  128,  162-163 

reduction  of,  to  ferrous  oxide 78,  150,  162,  163-166 


INDEX  OF  SUBJECTS  261 

PAGE 

Ferrous  oxide,  determination  of 11,  110,  118,  182-191,  245 

errors  in  determination  of 122,  128,  183-186 

oxidation  of 183,  185,  211 

Pratt's  method  for 190-191 

simple  method  for 110,  118,  186-190 

Filter,  fitting  of,  in  funnel 92,  98 

folding  of 92,  98 

incineration  of 103,  144,  158 

neglect  of  ash  of 44,  105,  159 

size  of 78,  91,  92,  152 

weighed,  not  to  be  used 101 

Filtering  flask 36,  98,  99 

gasket 35 

Filter,  macerated 51,  154,  158,  161 

Filter  paper 44 

Filter-tube 35,  99 

Filtrate,  testing  of 98 

Filtration,  description  of 90-101 

in  Gooch  crucible 99-101,  182,  204-205 

simple 90-98 

suction 98 

Fine  grinding,  oxidation  of  ferrous  oxide  by 183,  186 

Flame  of  burner,  size  of 78,  103,  104,  188 

Flasks 36,  98 

Fluorine,  determination  of 15,  112,  233-235 

estimation  of  by  the  microscope 7 

influence  of,  in  titanium  determination 55,  169,  235 

occurrence  of 20 

oxygen  equivalent  of 128 

Flux,  mixture  of  powder  with 85 

Fragments  of  rock,  loss  of 64-66 

Freshness  of  rock 6,  59,  61 

Funnel 36,  40,  91 

Funnel-support 44 

Fused  silica  apparatus 39 

Fusion,  various  methods  of 85 

with  calcium  carbonate  and  ammonium  chloride 196 

with  sodium  carbonate 85,  116,  131-137 

with  potassium  pyrosulphate 85,  117,  159-161 

Gas  generator 36 

Gas-washing  cylinders 36 

Gauze,  silk,  for  sieve 45 

wire 45 

Gelatinous  precipitates 90,  94,  142,  147,  149,  152 

Glass,  action  of  ammonia  water  on 48 


262  INDEX  OF  SUBJECTS 

PAGE 

Glass,  apparatus 34-39 

Glass-ware,  quality  of 34 

Glasses  for  colorimeter 37,  43 

Glucinum,  occurrence  of 21 

Gooch  crucible. 31,  99-101,  182,  204 

filtration  in 99-101,  182,  204-205 

Granularity,  influence  of,  on  size  of  specimen 62 

Grinder,  mechanical 66 

Grinding,  discussion  of 66,  70,  183-184,  186 

special,  of  powder 183,  186,  193 

"  Guarantee  "  reagents 45 

Haiiynophyres 13 

Hood,  operations  under 27,  87,  140,  146,  159,  187,  217,  221 

Horn  spoon 44 

Hot  plate 41 

Hydrofluoric  acid,  influence  of,  in  titanium  determination 55,  169,  235 

Hydrogen  peroxide,  reagent 50,  168 

sulphide,  detection  of 165,  236 

expulsion  of 165 

precipitation  by 224 

use  of,  as  a  reducing  agent 163 

Hydroxyl 12,  61,  210,  211 

Hygroscopic  water,  determination  of 12,  111,  116,  208,  243 

Igneous  rock,  average 125 

Igniter 41 

Ignition  of  precipitates 101-105,  143,  157 

position  of  crucible  during 103,  104,  144,  158 

Impure  reagents 45,  77 

Incineration  of  filter 103,  144,  158 

Inexperience,  errors  due  to 78 

Iron,  contamination  of  sample  by 65,  67 

errors  in  determination  of 122,  128,  162-163,  183-186 

ignition  of 117,  157-159 

metallic,  detection  of,  in  sample 67 

oxides,  determination  of 

11,  1C9,  117,  118,  128,  162-167,  182-191,  244,  245 

precipitation  of 117,  146-157 

precipitation  of  by  cupferron 162 

sulphide,  reagent 51 

Labelling  of  beakers,  etc 71,  74,  79 

Labels 44 

Laboratory 27,  73,  76 


INDEX  OF  SUBJECTS  263 

PAGE 

Lawrence  Smith  method  for  alkalies 110,  191,  192,  193-202 

Lead  acetate,  use  of,  in  detection  of  hydrogen  sulphide 165,  236 

oxide,  use  of,  in  determination  of  water 215 

peroxide,  use  of  in  determination  of  manganese 220 

Lime,  determination  of 11,  109,  117,  177-179,  245 

errors  in  determination  of 122,  177-178 

use  of,  in  determination  of  water 215 

Limits  of  error 124-126 

Lithia,  determination  of 14,  206 

Lithium,  occurrence  of 19 

Litmus  paper 51 

Locality,  choice  of,  in  selection  of  specimen 59 

Loss  on  ignition,  determination  of  water  by 16,  210,  211-213 

pulverization 64,  67 

Macerated  paper,  preparation  of 51 

use  of 154,  158,  161 

Magnesia,  determination  of 11,  110,  117,  180-182,  245 

errors  in  determination  of 123,  147,  180-181 

mixture 51,  218 

Magnesium,  use  of,  in  determination  of  potash 208 

pyrophosphate,  solution  in  nitric  acid 181 

Main  constituents 11-13 

portion  for  analysis 131 

Manganates,  action  of,  on  platinum 33 

Manganese,  coloration  of  cake  by 135,  243 

distribution  of,  in  analysis 14,  220 

occurrence  of 14,  18 

removal  of  stains  of 35 

Manganous  oxide,  colorimetric  method  for 220-223 

determination  of 14,  111,  116,  151,  219-225,  243 

errors  in  determination  of 14,  18,  124,  150,  220 

gravimetric  method  for 223-225 

standard  solution  of 51 

Manganous  sulphate,  influence  of,  in  iron  titration. 185 

Marble 51 

Material,  amount  of,  for  analysis 62-63 

Measuring  cylinders 37 

flasks 37 

Mechanical  grinders 66 

Meker  burner 40,  105,  179,  215 

Meniscus,  reading  the 107 

Metal  apparatus 40-43 

Metatitanic  acid,  precipitation  of 177 

Methodic  errors,  discussed 119-126 

Methods  of  analysis,  character  of 119 


264  INDEX  OF  SUBJECTS 

PAG3 

Methods  of  analysis,  errors  of 119-126 

general  discussion  of 109-129 

Method  of  weighing,  by  addition 129-130 

by  subtraction 130-131 

Methyl  orange 52,  151,  226 

Microcosmic  sail 55,  181 

Microscopical  examination  of  rock 6-7,  58,  60 

Mineral  composition,  estimation  of,  by  Rosiwal's  method 7 

Minerals,  brittleness  of 66,  71 

decomposition  of 84 

Minor  constituents 8,  13-17 

errors  in  the  determination  of 124 

Moisture,  adsorption  of,  by  rock  powder. 66,  72,  76,  208 

Molecular  numbers,  statement  of 24 

Molybdenum,  determination  of • 238 

occurrence  of 11,  21 

Mortar,  agate 43,  195 

steel 41-42,  69,  71 

"  N.d.",  use  of  term 25 

Nessler  tubes,  use  of 174 

Nickel,  determination  of 15,  224,  238-240 

occurrence  of 18 

Nickel  crucible,  use  of,  as  radiator 42,  145 

Nitric  acid,  reagent 47 

solution  of  magnesium  pyrophosphate  in 181 

solution  of  rock  powder  in 217,  232 

use  of,  in  dissolving  ammonia  precipitate 153 

"  Not  determined,"  use  of  term 25 

Notes,  taking  of 74,  83 

Number  of  operations  possible 115 

Occurrence  of  various  elements 17-21 

"  Opening  up,"  use  of  term 84 

Operations,  decomposition 84-87 

drying  precipitates 101-103 

errors  in 75-79 

filtration 90,  101 

general  discussion  of „ .  73-108 

ignition 103-105 

precipitation 87-90 

preliminary  observations  on 73-75 

titration 52-53,  166-167 

washing  precipitates 96-98 

weighing 79-84,  129-131 

Operative  errors 75-79,  119,  120 


INDEX  OF  SUBJECTS  265 

PAGE 

Order  of  constituents  in  tabulation 21,  23 

Ores,  origin  of 2,  10 

Osmiridium,  material  for  mortar 65 

Oven,  drying 42,  101 

Oxidation  of  ferrous  oxides  during  pulverization 183 

Oxygen  equivalent  of  chlorine,  fluorine,  and  sulphur 128 


Palau,  substitute  for  platinum 30,  132 

Paper,  macerated 51,  154,  158,  161 

wrapping  of  rock  in 68 

Penfield's  method  for  water 110,  210,  211,  213-216 

Perchlorate  method  for  potassium 202,  207-208 

Perchloric  acid 52,  207 

Permanganate  solution,  color  of 53,  106,  167,  185,  221-223 

standard 52,  54,  166-167 

Personal  errors 77,  119,  169,  172 

Persulphate,  ammonium,  use  of 151 

Phosphomolybdate,  ammonium,  precipitate 216,  218,  219 

Phosphomolybdic  anhydride . 219 

Phosphorus,  occurrence  of 19 

pentoxide,  correction  for  vanadium 14,  216,  238 

determination  of 13,  111,  116,  216-219,  246 

errors  in  determination  of 124,  216 

Physical  chemistry,  relation  of  petrology  to 1,2 

Pipettes : 37 

Plan  of  analysis 109-113,  116-118 

Platinichloride,  method  for  potash 203-207 

use  of,  instead  of  chloroplatinate 202 

Platinum,  apparatus 30-34 

attack  of,  by  reagents 33,  146,  150,  163,  164 

basin 31,  136,  140,  194 

care  of 32-34,  79,  132,  135 

chloride,  see  chloroplatinic  acid 50,  203 

cone 32,  98 

cost  of  potash  determination  with 202 

crucible .31,  79,  132,  135,  136 

foil 32,  165 

in  filtrates 146,  150,  163,  164,  205,  224 

loss  on  ignition 32 

occurrence  of 21 

precipitated  as  sulphide 164,  224 

quality  of 31,  32 

residues 207 

solution,  see  chloroplatinic  acid 50,  203 

substitutes  for.  .  30 


266  INDEX  OF  SUBJECTS 

PAQB 

Platinum,  tongs 32,  188 

triangles 32 

Plattner  "  diamond  "  mortar 41 

"  Policeman  " 37,  95,  155 

Porcelain  apparatus 39-40 

basin 40, 139,  140 

crucible 40 

plate 40 

Porphyritic  texture,  influence  of,  on  size  of  sample 63 

Portion,  main 131 

Portions  for  analysis,  number  of 109-113 

weighing  of 129-131 

weights  of 62,  109,  113 

Position  of  crucible  during  ignition 103,  104,  144,  158 

Potash,  determination  of 11,  110,  116,  191-208,  246 

as  cobaltinitrite 203,  208 

as  perchlorate 202,  207-208 

as  platinichloride 118,  202,  203-207 

apart  from  soda 208 

errors  in  determination  of 123,  192-193 

Potassium  bisulphate,  see  potassium  pyrosulphate. 

chromate,  solution  of , 50,  238 

nitrate 52,  135,  225 

perchlorate 202,  207-208 

periodate 220 

permanganate,  solution  of 52-54,  166-167 

platinichloride 118,  203-207 

pyrosulphate,  attack  of,  on  platinum 33,  163,  164 

fusion  with 85,  117,  159-162 

reagent 54 

thiocynate 54,  165 

titanofluoride 54,  55 

Precipitant,  addition  and  excess  of 88 

Precipitates,  adsorption  of  salts  by 90 

blasting  of 105 

characters  of 87-90 

drying  of 101-103,  104,  144,  158,  205 

gelatinous 90,  94,  142,  147,  152 

ignition  of 101-105 

impurities  in : 90 

removal  of,  from  beaker 95-96 

re-solution  of 90,  95,  148,  154,  178,  181 

washing  of 78,  90,  94,  95,  96-98 

Precipitation,  operation  of,  described 87-90 

Precise  methods  in  atomic  weight  determinations 75 

Preparation  of  sample . .  63-72 


INDEX  OF  SUBJECTS  267 

PAGE 

Pulverization  of  sample,  contamination  during 65,  66,  67 

loss  of  powder  during 67 

methods  of,  discussed 64—68 

operation  6f ,  described 68-72 

oxidation  of  ferrous  oxide  during 183 

Pyrex  glass 34 

Pyrite,  influence  of,  in  Mitscherlich  method 183 

oxidation  of,  in  fusion  with  sodium  carbonate 133,  225 

sulphur  of 15,  128,  225,  231 

Pyrosulphate,  see  potassium  pyrosulphate. 


Qualitative  examination  not  necessary 75 

Quality  of  glassware 34 

of  platinum 30,  32 

of  reagents 45-46,  77,  127 

Radiator 40,  42,  145 

Radium,  occurrence  of 21 

Rare  earths,  determination  of 225,  226,  229-231 

occurrence  of 18 

Reagents,  list  of 47-56 

quality  of 45-46,  77,  127 

Recalculation  of  analyses  to  100  per  cent 25,  61 

Reduction  of  ferric  to  ferrous  oxide 78,  150,  162 

References,  list  of 247-248 

Report  of  Committee  on  Analytical  Methods 121,  124,  125 

Representative  character  of  specimen. 3,  57,  63 

Re-solution  of  precipitates 90,  95,  148,  154,  178,  181 

Retort-stands , 42 

Rider,  use  of,  in  weighing 28,  83 

Right-handed  person 80,  93 

Rocks,  general  characters  of 1-3 

Rock-mass,  character  of 57-59 

Rock  powder,  decomposition  of 84-87 

special  grinding  of 183,  186,  193 

weighing  out  of 129-131 

Rubber  apparatus : 40 

Sample,  amount  of,  needed  for  analysis 62-63 

contamination  of,  in  pulverization 65-68 

preparation  of,  for  analysis 63-72 

pulverization  of 64-72 

selection  of 57-62 

special  grinding  of 183,  186,  193 


268  INDEX  OF  SUBJECTS 

* 

PAGE 

Sample,  use  of  all  of 71 

Sampling  of  rock 57,  63,  76 

Sand,  use  of,  in  cleaning  platinum 32 

Scandium,  occurrence  of 18 

Sea  water,  chloride  derived  from 20,  233 

Selection  of  rock  specimen 57-62 

Separatory  funnel 37 

Sieve 45 

use  of 67,  69-70 

Silica,  contamination  of  sample  by 66 

determination  of 11,  109,  116,  139-146,  162,  243 

errors  in  determination  of 121,  139-140 

evaporation  of  with  hydrofluoric  acid 140,  145,  162 

evaporation  to  render,  insoluble 116,  139,  140,  143 

extra,  with  ammonia  precipitate 140,  162,  243 

filtration  of 141-143 

fused,  apparatus 39 

ignition  of 116,  143-146 

impurities  in 140,  141,  145,  162 

necessity  for  double  evaporation  of 139 

removal  of,  from  porcelain 139,  140 

recovery  of,  in  ammonia  precipitate 140,  162 

separation  of 140-143 

Silk,  contamination  of  rock  powder  by 68 

Silk  bolting  cloth 45 

Silver  chloride,  precipitation  of 232 

nitrate,  solution  of 54,  220 

use  of,  in  testing  filtrates 98 

Sizes,  proper 74,  78,  91,  114 

Smith's  method  for  alkalies. 110,  191-202 

Soda,  determination  of 11,  110,  116,  191-208,  246 

errors  in  determination  of 123,  192-193 

Soda-lime 55 

Sodium  acetate .' 55 

precipitation  of  alumina  by 124,  149,  155-157 

ammonium  phosphate,  reagent 55 

bismuthate,  use  of,  in  manganese  determination 220 

carbonate,  action  of  on  platinum 33,  146 

decomposition  of 146 

fusion  with 85,  116,  131-137 

reagent 55 

chloride,  derived  from  sea  water 20,  233 

oxalate,  reagent 53,  55 

pyrosulphate,  use  of 54,  161 

Solution,  double,  of  precipitates 90,  95,  148,  154,  178,  181 

chloroplatinic  acid 50,  202,  203 


INDEX  OF  SUBJECTS  269 


Solution,  silver  nitrate 54,  220 

standard,  of  manganous  sulphate 51,  77,  221 

of  potassium  chromate 50,  238 

of  potassium  permanganate 52,  77,  166 

titanium  sulphate 55,  77,  169 

Sources  of  error 75-79,  119-124,  127 

Spatula,  platinum '.     31 

Special  grinding  of  powder 183,  186,  193 

Specimen,  representative  character  of 3,  57-63 

selection  of 57-62 

size  of 63 

tubes 37,71 

Spencer's  law  of  evolution 1 

Stability  of  standard  solutions 54,  77,  163 

Stains  of  manganese,  removal  of 35 

Standard  solutions,  deterioration  of 54,  77,  163 

solution  of  chromium 52,  77,  238 

of  manganese 51,  77,  221 

of  permanganate 52,  77,  166 

of  titanium 55,  77,  169 

Statement  of  analyses 21-26 

Steel,  contamination  of  sample  by 65,  67 

mortar 41,  69,  71 

plate,  use  of,  in  crushing  rocks 64 

Steiger's  colorimeter 43 

method  for  fluorine 235 

Stirring  rods 37,  88 

Stone  slab 45,  134 

Strontia,  determination  of 14,  110,  179-180,  245 

Strontium,  occurrence  of 9,  19 

Subtraction,  weighing  by 130,  194 

Suction  filtration 98 

tube 36,  93 

Sulphides,  influence  of,  in  ferrous  oxide  determination 185 

iron  in 190 

occurrence  of 20 

Sulphur,  condition  of 232 

determination  of 13,  15,  111,  225-227 

occurrence  of 20 

oxygen  equivalent  of 128 

Sulphur  dioxide,  reagent 55,  163-164 

trioxide,  determination  of 13,  15,  111,  231-232 

Sulphuric  acid,  reagent -.     47 

Summation  of  analyses 126-129 

Supports 44 


270  INDEX  OF  SUBJECTS 


Tartaric  acid,  use  of,  in  titanium  determination 176 

Temperature  of  drying 101,  209 

ignition 30,  104 

Test  solution  for  chromium 238 

for  manganese 222 

for  titanium 170 

Test  tube  holder,  use  in  ferrous  oxide  determination 188 

rack 45 

tubes 38 

Testing  of  filtrates 97 

of  reagents 45,  55 

Texture  of  rock 62,  63 

Thermometer 38 

Thorium,  occurrence  of 21 

Time  needed  for  analysis 113-118 

Tin,  occurrence  of 21 

Titanium,  occurrence  of 9,  17 

precipitation  of,  by  cupferron 162,  177 

standard  solution  of 55,  77,  169 

Titanium  dioxide,  colorimetric  method  for 110,  118,  167-175,  244 

determination  of 13,  110,  118,  167 

errors  in  determination  of 124, 168-169,  173-174 

gravimetric  methods  for 175-177 

reduction  of,  by  zinc 163 

Titration 53,  105-108,  118,  162,  166-167,  189 

Toluene  bath 209 

Tongs 32,  188 

Trace,  definition  of  term 24 

Triangles 32,  39,  104 

Tubing 38,  40 

Tungsten,  material  for  mortar 65 

occurrence  of 21 

Turbidity 94 

Uniformity  of  rock  mass 58-59 

United  States  Geological  Survey 3,  9,  22,  64,  85,  125,  131,  168,  192,  208 

Uranium,  occurrence  of 21 

Vanadium,  determination  of 14,  237-238 

influence  of  in  ferrous  oxide  determination 185 

occurrence  of 9,  18 

precipitation  of,  with  phosphorus 14,  216,  238 

Volume-burette 35,  106 

Wash-bottles 38 

Wash-water,  amount  of 96 


INDEX  OF  SUBJECTS  271 


Washing  of  precipitates 78,  90,  94,  95,  96-98 

Watch-glasses 38 

Water,  adsorption  of,  by  rock  powder 66,  72,  76,  208 

combined,  determination  of 12,  110,  210-216,  243 

combined  and  hygroscopic,  discussed 12,  208 

distilled,  use  of 56 

errors  in  determination  of 16,  123,  128,  209,  210-211 

hygroscopic,  determination  of 12,  111,  116,  208-210,  243 

Water-bath 42 

Weathering  of  rocks 59 

Weighing  burette 34,  107,  166 

Weighing,  method  by  addition 129 

method  by  subtraction 130 

operation  of 79-84,  129-131 

"  rational,"  discussed 84 

Weight  of  portions  for  analysis 62,  109-113,  129-131 

Weights 29,  80,  81,  82,  83,  84 

testing  of 30 

Weller's  method  for  titanium 168-175 

Wire  gauze 43 

Women  as  analysts 5 

Yttrium,  determination  of 229-231 

occurrence  of 18 

Zero-point  of  balance 28,  81-82 

Zinc,  occurrence  of 21 

use  of,  as  a  reducing  agent 163 

Zinc  oxide,  reagent 56,  234 

sulphide 224 

Zirconia,  determination  of 7,  13,  14,  111,  225,  227-228 

identification  of .  . 228 

influence  of,  on  titanium  determination 175 

Zirconium,  occurrence  of 17 


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